Yeah, but they're probably coming:
Ami Furi Koneko wrote:monthly charts being scored of pp would be kinda silly imo. also you don't need to play every diff to get max pp >_> just get #1 on the hardest map with 100% for that.
Making monthly charts PP based (if it's planned solution for this) will also require playing every diff to get max PP for each song, which isn't really fun.
And the thing is that with pp, you do need to play every diff, or risk being nickle and dimed by someone who did, because you can score pp on multiple diffs in a set (which is good and bad). This can be potentially reduced by using a very aggressive weighting curve to limit the number of maps that a player can count. Or doing the total rank score thing, and using only the highest pp from each set (but only for charts) is another way... but I'm not fond of it.
My big concern with using pp for charts is that it's a statistical method, and for stats to work well, you need lots of data... and there doesn't seem to be very many real competitors on the charts (look at the #1 and the #40 scores on a chart, if things were competitive they should be a lot closer than they are).
But since osu!mania is implemented with normalized scoring, there's probably no better way for it. Especially with the max score fixed even with mods... because the bonus for using mods will essentially only be given in pp.