[Daisuke]

Mages generally make that choice with their allocation of skillpoints.
After you finish grabbing most of the useful evoker passives, you're can only pick like 3 maxed out spells a 4th spell with a few points in it. If you intended on clearing dungeons, I can guarantee two of those spells will be Sear and Magaisendo with the 3rd spell most likely being Overload for PVP situations. That's usually how it is if you aren't following a specific character theme.
And evoker can grab a bunch of skill to have some elemental diversity but then they sacrifice someone of their damage potential because they didn't max out some passives.
LBs has more freedom but they don't have charge mind like Evoker and their abilities have status effects tied into them so it's important for an LB to actually be able to use their most of their abilities. It's kinda lame to have supportish mage class not be able to use most of their abilities and just walk around with phoenix, altera, and typhur cuz meta.
Summoners don't even care about skill slots so this won't mess with them.

It won't really encourage anything since hybrid building means sacrificing a lot in your main stat to get "okay" growths in other stats on top of using classes you wouldn't normally be touching just for the growths. No one is going to go through that trouble for what's basically a dice roll that has a good chance of going bad just because they lost some skillslots. If you want to encourage teamwork and innovation, this isn't the place to start.