My main reason right now are:
1. Even if I did decide to do so, it'd be a long time before I got to it.
2. It's a lot of work.
3. It smacks too much of acknowledging that some people are just too immature to exist on a site otherwise.
4. I'd rather the people in item 3 just weren't here. That's a wish I probably can't have, though.
5. The kind of traffic I'm aiming for (and having some success with) is comprised of people who really are just here to talk about stocks and conduct themselves in a way that makes it apparent flamewars just simply won't erupt with them. As usual, I cite the AIM folks as the prime example. They're an excellent example of "No Bull, Just Beef".
6. Like on RB, an Ignore feature can be and will be misused and harmful to the people who use it. For example "Everyone, put so-and-so on Ignore because they're lying about this tenth-cent stock that'll go to $100 soon." Though it's not my job to protect fools from themselves, I'm also not too keen on putting the bullets in the gun they're holding to their portfolio's head.
I really should've held off opining on this again, as I'm even less ready to debate and analyze it than I am to do the actual work of implementing it.