The 2/3/4 island mechanic is something we discussed a lot during the beta.
In the end, the devs want you to be able to control how long or short a game is. If you want a short game, you can have one. If you want a longer game, you can have that too.
If you just like getting victories, you can get them pretty quickly in a short game. If you like having high-powered mechs, then you can have that too by going for a long game.
If there are certain islands that are tough for you, you can skip them. If there are some vek on an island you'd rather avoid, you can just not do it. If you aren't having fun with the squad you're playing, you can opt to end the game early.
In the end, it's an option that gives you more flexibility about how you play the game, and that's a good thing. If you don't like playing the short game, then you don't have to!