I wouldn't mind seeing this feature, but let me play devil's advocate. Here is how I see it.
Advantages of group chat being built into Osu!:
- It's convenient while playing if you only have one monitor
- You can click people's name to go to their profile
- You have /np
- It's easier to talk to people you met on Osu in the first place
- Don't need to run another program too
Advantages of other chat solutions (varies by chat client but I'm sure you can find one with a combination that works best for you):
- No chat censoring*, no after-the-fact message redaction, and you get to set the rules. Depending on the chat system you use, you could even appoint your own moderators.
- If you close Osu or it crashes, your chat is still there. You can still use it without having to restart Osu. You can say "Sorry brb, restarting osu"
- You can send files (e.g. a zip of some beatmaps you're about to play through)
- You can keep your group even after everyone leaves**
- It won't lag your game if someone uses a character from a font Osu hasn't loaded yet
- You can write bold, underlined or coloured text
- It will integrate better with your IME if you use one
- You can choose a better font (a big one for me, because Osu's font rendering doesn't do fallbacks correctly under wine. Probably less important for everyone else.)
- You can write scripts/bots/macros
- You can change your nickname at will
- You can call each other up (voice chat)
- TLS or other encryption can be used
- Less load on peppy's servers :^)
* If you don't think that's a problem, check these out:
it edits a URL and obscures the name of a tech convention;
it obscures a song name and turns someone's sentence into nonsense;
it overwrites someone's username. There are many more examples of this. "It could happen to you!"
** IRC, which Osu's chat seems to run on top of, has the rule that once everyone logs off a channel it disappears (with some exceptions -- chanserv). Trying to prevent this while not having tens of thousands of dead channels would be one hurdle that Osu devs face if they implement this.
I think the first 2 reasons alone are a pretty big reason to use an external chat client. If you have 2 screens I recommend you have some IRC or mumble chat going on one of them with Osu on the other. I do this while still using Osu chat in-game and it works well for me.