Depends... if you're in a serious debate about a serious topic with someone who isn't a sophist asshat and can back up his statements, then online debate is a totally valid option to test your theories and ideas against what others believe to see if they stand up to scrutiny.
If, on the other hand, you're getting drawn into some sort of prepubescent anonymous dick swinging contest, then yeah, it's an enormous waste of time.
The trick is learning to tell which one is which in time to not waste YOUR time.
