Lael normally uses more guardians, from what I've seen of his other armies. This one is trying something a little different, and moving away from the tw max-size troops squads that everyone feels compelled to take these days, because of the RT guidelines.
I think it was a choice between a Dire Avenger squad, or a Wave Serpent for the Banshees. Personally, while I love Wave Serpents, I think that the Avengers could work well here.
Taking a wave serpent makes it too tempting to send the Banshees out a turn too early, which leads to a lot of problems - dead banshes, for a start. Having a pair of units advancing, each supported by a secondary unit (Avengers or Storm Guardians) allows the army to stay together much better. Plus, the combination of a screening unit and Fleet of Foot is enough to close with most enemies pretty fast - the only time it would be a major problem is against static shooty armies. In this case, a lone vehicle (I don't count the vyper) would be shot down long before the Banshees were in charge range, as the Wave Serpent would provide the only large target to all those guns...
There are more reasons than 42 BS4 shuriken shots per unit, behind choosing to play Ulthwe... and a unit of warlocks are a pretty good reason. Plus, with three squads of guardians, the army has more of these troops than most Codex: Eldar lists, and certainly more than most other craftworlds. I like it...
One point about the storm squad - the aim seems to be to get the Banshees in combat. And casualties caused by the guardians are a bonus. Why not give them Conceal or Embolden, to ensure that the squad stays in front of the Banshees for as long as needed?