[Quick]

It depends on how you apply them.
They don't fill a 'heavy support' choice in the traditional sense, but they can certainly win back their points. The ShuriCat puts out a lot of firepower and against enemies that aren't wearing Power Armor, it rips them to shreds.

When playing against Orks or Tyranids, for example, I make sure to take a full squad of them, with a Spiritseer (with Destructor). The sheer number of shots will thin out those mobs so that the Nob armed with a Power Klaw cannot hide behind thirty meat-shields when his squad assaults your Wraithlords. Fleet of Foot will see to it that they never have to get in close combat.

Whether or not they make their own points back is not as important when they do more to prevent the enemy's units from making their points back.

Myself, I've recently grown to love Dire Avengers. I prefer to take them when I am outnumbered, but no so dramatically like when facing the Orks. (I field them against the Tau, for example) They do the same thing I ask of Defender Guardians, more or less, and they tend to do it better, I feel, despite giving up the some of the shots.