Is there a further benefit to gathering coins in Super Mario Galaxy 2 beyond getting an extra life for every 100 coins you gather? I know that in at least one level, you need coins to convince a Luma to transform, but that only takes the coins you gather in that level into account, not your total coin count throughout the game.
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.
Yes, there are a few ways that coins are actually useful. Here are a few of the reasons why it is good to collect and save coins. . .
There is a certain Luma called a "hungry Luma" who you may encounter on the level maps. There are two types of Hungry Luma: The star-bit Hungry Luma and the coin-eater Hungry Luma. The coin eating Hungry Luma wants you to feed it a high amount of coins. If you do feed coins to it, it will open up a secret level!
As you mentioned in your question, you do get an extra life every time you get 100 coins, but the more you play you will NOT be taking extra lives for granted. You don't lose your extra lives each time you exit the game, so stock up on 1-Up mushrooms while you still can!
I hope these suggestions helped you out!
If you get 9999 coins then a ghost of Luigi will appear on each level for you to race against. These are Nintendo staff times and will put your Mario skills to the test. The other way to make the Luigi staff ghosts appear is to complete each level as Luigi but this is very time consuming and it is easier to just get the coins.