As to why the Texans didn't trade down (beyond the obvious and probable reason that there wasn't a willing partner), the thing that makes the most sense to me is that they wanted a CB who would be able to start from day one, and believed that Jackson was the best, and maybe only, way to achieve that goal. Total speculation on my part, but with 4 CB's taken from their pick to the end of the first round, I'm not sure you can get comfortable that any specific player will still be there if you trade down more than one spot.
As to the idea of trading down in general, obviously, sometimes it's great, but let me say this about the two teams that were far and away the most active.
The Broncos traded numerous times, and pretty much ended up with Tim Tebow as the net result of all of that. Time will tell how things work out, but if I were a Bronco fan, I'd be even more ticked off than the last year has already made me. I do not view trading down as a winning strategy for them.
The other most active would be the Patriots. I thought trading down was a strategy to accumulate more picks because you had so many needs, and one or two stud players wasn't going to get you over that hump. That doesn't strike me as the Patriots. Granted the strategy is similar to recent drafts, but I don't think you look back on the recent Pat drafts as doing all that much to improve their team once you get past Jerrod Mayo (a #10 overall pick) anyway).
I like what the Cowboys did, but it was one specific move. I don't like what the Broncs or Pats did at all.