Depends on the amount and specific interval of time investment. MMOs create demands like playing 3-4 hours every tuesday and thursday evening without fail for raids. This now requires a 6-8 hour time investment a week minimum with strong social pressures from online friends. That's not going to be helpful if you have a test on wednesday or friday. If you had a normal RPG or other hobby you could pick it up and put it down without social pressures and regular time investments at intervals in the middle of the week.
Given that I know plenty of people who play MMOs and participate in raids while still managing to pursue their studies/work/parenting/etc. successfully, this description of the absoluteness of the MMO time demands seems a little overblown.
Granted, I don't actually play them myself, but it seems hard to believe that you couldn't have a good time playing an MMO while also finding an in-game social group that was reasonable about participation requirements.