Disabling an Apple ID would also disable its use in the online Apple Store and the OS X App Store, though. Seems that you might as well just disable iMessage. It's not like it provides that much added value over text messages.
Depends on your particular plan (unlimited text vs. sufficient data), access to Wi-Fi, and how frequently your friends send images or media over iMessage. I have a friend who turned of iMessage to reduce data usage.
