Coins are easy to ignore if you never do restarts, and if you use Rock Band World to set goals and to compete in Score Wars on a regular basis. If you don't want to do those things, the coins force you to do them anyway. It feels very natural for me to do all of those things automatically, but I totally understand why it would be annoying to be "forced" to play like me.
The coins were a really great idea which turn out to be hard to implement into the game smoothly. Hey, game theory is hard, especially when you try to add a dynamic monetary system. It gets even more complicated when you can use that currency to trade with others. The game EVE Online has a real economist to try and keep the market functional: