I have no idea where you read that Barmes is not as good as Manzella, defensively. I have not watched him regularly, but I have watched him play SS. And he was better than Sanchez and I'd say as good as Manzella. I'm no expert, obviously, so take that with a grain of salt.
However, as far as Manzella, his "high noon" was last year as well. His last year in AAA, the org had to do the same thing to light a fire under his ass. Something tells me that the Astros taking this effort is a good confirmation that every possible effort was taken to let Manzella earn the job and he didn't. I am reading into it but, to me, this trade is a clear indication they are not willing to go into ST with Manzella and Sanchez as the only two options. That's as much of an indictment against a player as you will get. Will Manzella ride the pine? No clue. But there is now experienced SS on the roster who poses a very serious challenge to both Manzella and Sanchez's playing time.
I didn't read a direct comparison between Manzella and Barmes anywhere- its just the impression I got, mainly from:
1. the way Astros brass have praised Manzella's defense, consistently
2. Mills told Wade (paraphrasing here) he thought Barmes could play SS every day, but that he is a plus defender at 2B.
But admittedly, that is fairly vague info to go on. I haven't seen Barmes play SS myself (not that my judgement would matter at all).
When did Astros brass say that 2010 was Manzella's "high noon"? Don't forget he was injured for a good part of the year, and, as Ron Brand pointed out, played better after he came back. I'm not saying he's Adam Everett, or even that I'm convinced he's a MLB starter. It just seems sort of strange to give up on him now, when they are committed to a youth movement.