Are coins used for anything other than Plant Food and Power-Ups?
Per the official Support page (click on the "How do I get more coins?" question):
You'll use the Coins to purchase extra Plant Food and Power Ups!
That's all they are used for.
I noticed recently that later in the game you can also spend coins to level up your plants and some plants can be bought with coins from crazy daves shop (also known as twiddydinkies.)