One possible interpretation of Iran's behaviour is that while it is certainly stirring things in Yemen it is doing so on the cheap, perhaps as a diversionary tactic. In the words of former US ambassador Barbara
Bodine:
"I think the Iranian interest in this is that the Saudis, as we know, are very much involved in trying to unseat Assad, who is extraordinarily important to Iran, and Iran coming in [to Yemen] and providing support to fellow Shia is a way of distracting the Saudis – and in that sense the Iranians have been terribly successful because we have reports of 150,000 Saudi troops on the border, large numbers of aircraft and ships and everything being pulled into Yemen.
"Everything that's being pulled into Yemen [by the Saudis, etc] is not being focused on Syria or, for that matter, ISIL. So, as an Iranian gambit to pull the Saudis away from what they consider more important, it was a very good gambit."