I wonder how much of this is influenced by what happened when Hezbollah captured one (or more?) Israeli soldiers at the northern border of Israel. I agree, I don't Iran is doing this just to get back their agents.
I don't think it was the primary influence for their move as Iran has pulled this stuff before ala'9 the 1979 hostage crisis and in 04' they snagged a # of British marines and they got away with both those maneuvers (but at the cost of becoming an international pariah state). However, the Hezbollah move last summer did illustrate that a stunt like this may invite a survivable limited conflict where just surviving equals victory