It's not that you should never map to vocals ever, but it's somewhat discouraged. Vocals are very difficult to map well most of the time. They're often a bit early or a bit late, sometimes use very awkward timing, and in general end up being hard to map accurately and well.
I want to talk a bit about how this applies to some of the harder charts out there (and to explain a bit about what tthey're doing, since some people see these charts and just think they're over mapped). There certainly are some overmapped charts out there though...
Here's an example video so I can point out mapping techniques at different times:
https://www.youtube.com/watch?v=IKQyoW5HwJ4
from about 18 seconds in to about 1 minute in you see some layering. You'll notice there are lots of 2 note chords there, it's pretty common on higher difficulties in o2jam for mappers to use chords to represent single sounds (like a kick drum) with single notes layered to represent the other sounds. At 1 minute in the holds begin and you can see layering a bit more clearly there since the holds are mixed in with the other notes. At about 1:11 you can see how the holds stay on the left while the single notes are on the right. There's also some layering on the right with the shorter holds mixing with the single notes while the long holds are still on the left. The patterns after that section are made up of layers, every beat you're hitting a chord, and along with that every half beat you're hitting a note from a short stair (or other pattern, not all of them are stairs). You'll also notice that the patterns aren't fully symmetrical but are still well balanced between both hands. When the holds come back at 1:48 you'll see yet more layering where the patterns move all around the holds. The holds also tend to alternate hands because once again, it balances the map out (although there are plenty of times where you could easily bracket the holds and the notes instead). At 2:09 or so you can see an interesting symmetrical pattern. In a few places Yumel employs something I'm a big fan of: irregular stairs. Instead of being a straight line where every note is directly beside the next one, there are small "holes" in the stairs (often used in a black and forth sweeping motion). This is a way to make things a bit harder to get good accuracy on and also lets you control how many notes you will use for each "sweep". Since 7k has an odd number of keys, a full sweep left to right would be 7 notes, and 4/4 music doesn't work well with odd numbers.
You'll probably also notice that in many cases there are holds that don't necessarily make sense (switching from 1 hold to 2 holds for a sound, etc.) This is fairly common on more difficult charts, and personally, I have no problem with it, except that it's very hard to tastefully pull off. The hold section at about 4:40 is an example of something that might be technically "wrong" if you get too strict, is still incredibly fun, and still sync'd with the music (the length of the holds is very regular they are all exactly 1 beat long). Personally, I think that bending the rules a bit can make for some excellent charts, but there's also the potential to be worse.
This for instance, would break a few rules: https://www.youtube.com/watch?v=qi23wknsz-4 But it's still one of my favorite maps that I've converted from o2jam. As a bonus at about 4:17 there's a section involving "reverse mapping" or "inverted mapping" where instead of notes you have "lifts" where you are holding all keys and must lift then press a key where a note would be. This is very difficult to play (I can still only survive very short and simple inverted sections).
I want to talk a bit about how this applies to some of the harder charts out there (and to explain a bit about what tthey're doing, since some people see these charts and just think they're over mapped). There certainly are some overmapped charts out there though...
Here's an example video so I can point out mapping techniques at different times:
https://www.youtube.com/watch?v=IKQyoW5HwJ4
from about 18 seconds in to about 1 minute in you see some layering. You'll notice there are lots of 2 note chords there, it's pretty common on higher difficulties in o2jam for mappers to use chords to represent single sounds (like a kick drum) with single notes layered to represent the other sounds. At 1 minute in the holds begin and you can see layering a bit more clearly there since the holds are mixed in with the other notes. At about 1:11 you can see how the holds stay on the left while the single notes are on the right. There's also some layering on the right with the shorter holds mixing with the single notes while the long holds are still on the left. The patterns after that section are made up of layers, every beat you're hitting a chord, and along with that every half beat you're hitting a note from a short stair (or other pattern, not all of them are stairs). You'll also notice that the patterns aren't fully symmetrical but are still well balanced between both hands. When the holds come back at 1:48 you'll see yet more layering where the patterns move all around the holds. The holds also tend to alternate hands because once again, it balances the map out (although there are plenty of times where you could easily bracket the holds and the notes instead). At 2:09 or so you can see an interesting symmetrical pattern. In a few places Yumel employs something I'm a big fan of: irregular stairs. Instead of being a straight line where every note is directly beside the next one, there are small "holes" in the stairs (often used in a black and forth sweeping motion). This is a way to make things a bit harder to get good accuracy on and also lets you control how many notes you will use for each "sweep". Since 7k has an odd number of keys, a full sweep left to right would be 7 notes, and 4/4 music doesn't work well with odd numbers.
You'll probably also notice that in many cases there are holds that don't necessarily make sense (switching from 1 hold to 2 holds for a sound, etc.) This is fairly common on more difficult charts, and personally, I have no problem with it, except that it's very hard to tastefully pull off. The hold section at about 4:40 is an example of something that might be technically "wrong" if you get too strict, is still incredibly fun, and still sync'd with the music (the length of the holds is very regular they are all exactly 1 beat long). Personally, I think that bending the rules a bit can make for some excellent charts, but there's also the potential to be worse.
This for instance, would break a few rules: https://www.youtube.com/watch?v=qi23wknsz-4 But it's still one of my favorite maps that I've converted from o2jam. As a bonus at about 4:17 there's a section involving "reverse mapping" or "inverted mapping" where instead of notes you have "lifts" where you are holding all keys and must lift then press a key where a note would be. This is very difficult to play (I can still only survive very short and simple inverted sections).