Even if you're not limited by Hit Dice, the Ruin Swarm doesn't look all that great for wildshaping. You keep your old type, which means you don't get the Swarm or Vermin type benefits. You gain Ex attacks, but not special qualities (so no fast healing or Blindsense). You keep your own hit points and base saves, so you don't get all those HP (your saves do go up a bit from high ability scores though)
I do have to admit that although the 5d6 damage is pitiful, the blinding and distracting attacks are rather good. They offer saves, but they're of the "you can't beat this" variety which means you're almost guaranteed to keep at least 1 enemy locked down forever, while you still have all your spellcasting (you did take Natural Spell, right?). Assuming you came in with Druid, you trade your Animal Companion progression for a craptacular Vermin Companion, and your Wild Shape progression for a slightly accelerated but less versatile one, culminating in the Swarm Form ability. You keep 10/10 spell progression.
