1) An elite pass rusher won't be available at 30, or even in the low 20's (which is the furthest we could trade up without great sacrifice). All we might be able to get is someone like a cover LB with modest pass rush skills (e.g., Ayers) or a stout 34 DE who is capable of an occasional sack, but is no one an opponent would need to scheme for (e.g., Heyward, Wilkerson, Clayborn), so if we're looking to optimize value, picking Smith, whose upside might be close to Revis would be more attractive than a solid 34 DE, whose upside might be someone like Shaun Ellis.
2) Ryan has a touch of hubris (or call it stubbornness, or just plain deep belief in his system). His system calls for elite CB play but unexceptional (that is to say, solid) DL play based, among other things, on the notion that if receivers are covered, even average front seven players will be able to get pressure on the QB if the proper scheme is called. So if we weren't able to get enough pressure on the QB last year, it means (to Rex) one of two things: either the CBs didn't do their jobs well enough (so all the more reason to go after someone who might become an elite CB, particularly as Cromartie is more than likely gone) or the scheme needs to be tweaked. It won't mean we should expend a high draft pick on a front seven guy.
Another possibility is to see if Carimi is there at 30. Vlad is not happening at RT.