The good way to do that would make enemies varied that need to be dealt with in different ways, the bad way would be to limit your options to deal with them and then force you to make optimal choices for all scenarios in your 8 ability slots instead of extremely optimal choices for certain scenarios.
That would work but it still wouldn't encourage you to think about party synergy as much. The point is to specialize for each character and make sure the party on the whole is a force to be reckoned with.
Technically this just spells out to you which abilities to pick, though. So its not like it is making the game more tactical (wich they claim was the intention behind it).
In-battle it doesn't, but rewarding players who pay attention, read the codex, listen to what the advisors are telling you, listen to what scouts are telling you...that does make me think more.
For example before I knew about this limit how important did the scout telling you about undead and Avvar actually seem? Not very, because you can use all your abilities. Now, that scout's info seems important. With an ability limit, you now have a higher opportunity cost for skipping research upgrades (maybe). Things like that.