I don't know if the distinction is so much between "important" and "unimportant" as it is between "building games that amuse people" and "building games that function as electronic Skinner boxes."
I don't think it's so much "important" vs "non-important" as "regular business" and "evil". Zynga is known for spamming you with friends request, cheap gambling simulators such as *Ville, Mafia Wars (iirc), stealing from anyone they couldn't buy, etc. And lately their "offers" have been getting more and more gray, or so I heard.
My logic is that if you went to go work for Zynga and consciously ignored what you were working on (and the ethics therein) because of hopes for the big IPO exit, you are now left with no money and the last several years of your life spent on something you regret. I'm not making any statements about my opinion on Zynga, but based upon the press and popular opinion here, you can be sure there were engineers who turned a blind eye to what they were working on in hopes of striking it rich.
My point is that regardless of the monetary prospects, how you choose to spend your time matters, because the only certain thing is what you are working on now, not the payout in the future.
I don't follow your logic here.
You're implying that the work done at Zynga is non-important, or is some how bad.
But plenty of smart people work at "important" world changing places and never hit huge success.
Plenty of smart people work at "non-important" places and end up hitting it big (like most year-old startup being acqhired these days).