Is there a point to collecting coins in Super Mario Galaxy 2?

There is but one good reason to collect coins. Whenever you turn the game off, you lose all your extra Marios and are reduced to the default 4 lives. One of the Lumas down the pipe on Starship Mario will let you roll five Chance Cubes (for a possible total of 25 lives) for 100 coins.

This comes in especially handy at the very very end of the game when you get to the Grandmaster Galaxy. First-time players may well go through thirty or forty lives (or more) before they can claim the very last star of the game.

Excepting the occasional hungry-for-coins luma, there is no reason to go out of your way collecting coins in Super Mario Galaxy 2.

Your coin collecting will never trigger stars, unless you're collecting purple coins to begin with, which are far from the average case.