I can imagine that would motivate, yes, but for me the point of side projects is to explore things that are less commercial, more researchy.. things BigCo wouldn't pay me to do. And to do something commercial while at BigCo would require quite some paperwork for BigCo to agree. And it is not like I need to be doing commercial work on the side for financial reasons.
I've managed to get myself into what some might consider an ideal situation: a half-time job at BigCo. It means my work at BigCo is more interesting and condensed, and of course it means I have all the time in the world for side projects. BigCo total comp is so incredible that half of it still pays the bills in ExpensiveCity.
Except all the challenges echoed by others in the comments are real: overcoming procrastination and doing focused work. Actually the former I can manage, the latter is harder. All I have to show for side project time is a long list of smaller projects, I can't manage to take one of them and build/polish it into something significant. Somehow having the choice 100 fun project makes devoting time to just 1 super hard.
Maybe half-time off to some extend is worse for procrastination than full-time, since I can always fall back on my work at BigCo as a sign of progress in my life.
This situation should be ideal, I know the path I should take, the opportunity is there for the taking, but so far have not managed.