Super Mario Galaxy 2

One question though, in SMG2 do you unlock Luigi after getting all stars with Mario like the first one? And what is the point of choosing to play as Luigi in those levels where he appears at the beginning?

You don't need to get all stars with Mario in Galaxy 2, if I remember right. It's an option later on without needing to find them all. And I guess choosing to play with Luigi's just a fun little option for fans or people who feel playing with Mario's too easy (Luigi slides and runs out of air quicker when spinning under water, plus he jumps higher).