That's because he's not very good. Welcome to backup QB in the NFL.
People aren't so much defending Weeden in these threads in the first place. They're trying to help keep things in a relevant context. They guy can't handle pressure, he doesn't look anybody off. He doesn't process reads very quickly. He checks down to receivers who don't have a high likelihood of converting. He can't get you out of 2nd and 15, for the most part. He lacks touch over the middle. He's a very limited QB.
What he can do is stand in the pocket and get the ball our accurately. So far at least, he's avoided the really stupid turnover. The reads he does make tend to be open receivers, at least. It's a fairly short list.
Which is what happens when you lose your franchise QB. You get a guy who's really limited or, if you're even unluckier, you get a guy who can't do anything at all for you.
I don't like Weeden. I didn't want him as the backup going in. So far, I've been very happy with what we've gotten relative to what I expected, but that's because my expectations were very low. I'm not about to waste time ruing that he's not Matt Hasselback, though.