If people want to stack 10 AP then I suppose that's their prerogative. I don't even think that's a bad thing if they're level 20 and have the attribute points for that, as you'll probably have the skills to make use of that by then.
But having 10 AP would take 16 attribute points, and if you don't have any points in Strength then moving 4 over would net a 40% damage increase. Or if you don't have any in Aptitude, then with the way Grit/Willpower works all your statuses will probably apply a strength of say, 4, and if the enemies have a Grit/Willpower of 4 then all your statuses will apply a strength of the minimum of 1. That means it would take 8 casts of a knockdown to knockdown a single enemy. Moving, say, 5 points from Dex to Aptitude would increase the strength of your knockdown status by 50% and, increasing it to a total of 6, making it take 4 casts of a knockdown to actually get them down.
So I don't think that maximum AP will actually be all that useful.
But as I said, if people really do want to make the sacrifice for that much AP, then that's their prerogative.
I don't think that's necessarily a bad thing. And I don't think it's necessarily a bad thing if people have 10 AP by the end of the game, so long as enemies are tough enough to warrant that.