Like that "benchmark" showed (I Know you loved them stragy), with bigger images, the different in size can be very big.
I could upload if you want and I can bet you anything that you would not be able to distuingish the PNG from the JPG12 (not even from the jpg11 or jpg10).
On a osu background at 1280x1024 (Which is max I beleive?), the different will easily be in the 1-2mbs, which in a song file, can be alot.
Ofcourse, for smaller images, size isn't important at all, thats why all my signatures are PNGs. But when you make a site background with a big gradient, you usually don't want a 2-3mb file.
Same goes for FLAC. I would never use it for a 3hour conferance speech with no music in it. Each format has it's own use. There's a reason why if they are all still alive.