I agree on the short starts thing, and that still kind of exists even with the opener if you're using an Archer or Bundy to piggyback the opener. That 3 inning start just occurs one inning later, getting you to the 4th.
I'm not a fan of the opener concept, it's not "baseball" to me. But, to me, it doesn't expose the bullpen more than it already would if the starter has a short start. It just means that the guy that would have pitched that 4th inning is now pitching in the 1st inning instead. Everything else stacks up behind that just as it would had the starter only gone 3 innings and that same reliever pitched the 4th in a conventional situation. The bullpen still has to pitch 6 innings of the game either way.