Yes, it makes you more productive, but what if you fall in love, get sick, have a child...? Then you feel guilty about not catering to your side projects and guilt breeds procrastination.
I learned how to break down work into small pieces and rather finish one small piece and then call it a day instead of leaving something half-working for the next day. Because of this, I left projects dormant for 3 months and then picked them up again.
Granted, my side-projects are for-fun and not for-money, that makes it easier...