Leveling up other classes to get the extra abilities that cross over takes like...2 days of fate grinding. Far as I know for WHM, you only need to get ACN to 34 and THM to 24 to get what you need so one does not need to cap those out; correct me if I am wrong.

Personally, I am not an 'alt' person. I find a role I like and stick with it for years on end usually.