Hi Justin,
I'm one of the cohosts of PBPitch so I thought I would chime in.
First, I definitely agree with what Sarah and June said about it being more than just a way to catch an agent's eye. It's a great way to build your writing community and a great way to just hone your pitch!
Second, we invite legitimate agents to each event, but since it is a public event and anyone can participate, yes, there are vanity presses and "schmagents" who will stop by and favorite pitches, so do go in with your eyes wide open and be prepared to thoroughly research before you submit if you get a favorite.
And sometimes an agent will tell us they hope to stop by and then they don't. Life happens, they are busy, something came up, or frankly, they stopped by for a few minutes and nothing caught their eye. I do think with the proliferation of Twitter pitch parties the last few years, they are getting more "selective" in their favorites. And yes, possibly tired of the pitches they see over and over again as both writers and agents participate in several events.
We have had success stories from every single event where writers signed with an agent or a publisher based on their PBPitch. But of course, it's not the only way to find an agent. I found my agent through a slush pile submission, though seeing her "hearts" on our event last February helped me know what she liked. . .
Also, the feed moves by SO fast and their are thousands of pitches. I know you said that you only saw three agents for the last event in October, but a few of us watch the feed throughout the day as, at the end of the day, we like to "thank" the agents and editors who stop by, so I do know we had at least 8 or 9 agents stop by. . . again, they may be getting more selective in their likes, but they do stop by.
If you have any more questions, don't hesitate to post here. And good luck if you decide to join the party on February 22nd!