My opinion: Muslim rebels that will impose a Muslim theocracy is worse than bad guy secular dictator. We saw that with Iraq, Libya, and other Middle East Countries. You might even throw Iran in there, although I don’t think the Shah was that bad. Neither side in Syria is good, but one is worse than the other. And as bad as Assad is, he at least protected the Syrian Christians (as far as I know).
More than anything, the Shah was corrupt. So that prevented Iran from developing economically.