Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General Please Clarify Ability, Max Skills and Skill Level

I do have another question towards ability levels in skills:
where for example is a point in having 5 levels in scoundrel other than having unlimited number of scoundrel skills (personally I think 7 is more than enough). The highest scoundrel skill wind up toy (lvl 13) would just need scoundrel lvl 4 to have no AP penalty. For all other scoundrel skills rank 3 would be enough.
Anyway I prefer my hybrid rogue (air scoundrel witch) , but I can see no point in investing pure scoundrel (well true, the game never has pure "classes"
...
anyone can enlighten me? Please.

edit: spell checked, sorry for my mobile device and confusing terminology (ability rank in skill vs. Skill level which determines AP penalties)