A Nazi made out of bees. That right there is what we in the business (plumbing) like to call a 'high concept'.
Swarm has pop-culture cred. He's a ridiculous concept dressed up in silver age nonsense with a ready-whip topping of pure cracked out awesome. Nazi made out of bees. It's that simple.
Swarm's powers are about what you'd figure, what with him being made of bees and his weaknesses are derived directly from those powers, which makes him a nice package villain. He's a done-in-one baddie, and makes for some interesting visuals with the right artist.
Too, he can't really be destroyed, not by comic book logic, and he's got no attachments as far as opponents. He's an anywhere/anytime sort of villain, with a creepy vibe (Nazi + insects=disturbing), and the ability to give most heroes a run for their money. After all, how do you fight a swarm of bees? Besides with a water hose, I mean.
The only problem with Swarm is his lack of motivation. He's a sort of natural disaster on legs, with no real reason for his actions beyond a sort of 'Empire of the Insects' thing he has going on in some of his appearances. Of course, in the hands of the correct writer that could translate into something solid, story-wise.
These days, I think he'd make a good reoccurring nemesis for the newly reinvigorated Hank Pym...ants vs. bees anyone?