If you read the minds of the people you're supposed to chose from you'll find out their strong points. For example one necromancer can help your creatures magic resistance, the other melee resistance and so on.
The deep implications I still don't know them because, for example, I rather buy my healing potions than use my alchemist to make some. In fact another good example would be the alchemists. One is the best alchemist there is and the other is a complete beginner (you'll find this via mind read or book entries) So I chose the beginner one but he's able to make a cool potion that boosts my resistances A LOT (atm I don't know if that's permanent)
Experiment a little, but don't let the island choose for you. You'll miss the experience and the fun =)
