Is there a problem with having diversity in the market? Whilst I can always wish that every development studio were developing games to suit my needs and tastes; obviously it's far better that there are a range of studios producing a range of titles, a large number of which I won't like. I might not enjoy the games that Zynga produce, but it seems pretty clear that there are plenty of people out there that do enjoy them very much; and I'm not sure that more people "playing" generally is a bad thing.
More people playing more games from more companies is only ever going to be a good thing for the industry.
My feelings on this are hard to put into words -- Zynga is the equivalent of churning out crappy romance novels, soulless pop music, kitsch art, or even junk food (made to be addictive, incidentally "nutritious").
"Art" is undefinable, but one heuristic is something genuinely meant to evoke feelings in the audience and perhaps teach them something as a result.
For me, it comes down to intent. Zynga & friends are building "compulsion loops" which aren't meant to entertain, they're meant to addict. I don't have a problem with a diversity of games, everyone has different tastes, etc. but... some things just feel skeevy.
I'm no fan of Zynga and their work (as a gamer), but they have many, many (many!) people who enjoy playing their games. That alone is reason enough that should exist.
Zynga have been a massive factor in the evolution of video games, for better or worse.
More people playing more games from more companies is only ever going to be a good thing for the industry.