CapnZapp: In this thread you haven't gone into a lot of detail, but you've outlined the numerous feats you'll be banning in another, so it's fair game to say you went on a "banning spree" with the PHB 2 feats.
As for this feat. First off, yes, against a single medium opponent only two people can flank...if they're standing still. As was already demonstrated with the flanking dance, you can get around this. Also, with large or larger creatures it's pretty easy to set up at least 3 people in flanking positions. Heck, it can even be advantageous since it limits the movement of the creature.
As for the viability of a melee Rogue, this doesn't affect it. For a party that has no Rogue, neither build is viable. For a party made of nothing but Rogues, either is viable. It all depends on party composition. In my party, for example, our Rogue has said that he won't be taking this probably (even though I think he would get some use out of it) because we don't have a lot of melee players. There's a Fighter and a melee Cleric, but we're usually trying to form a wall and almost never actually flank with each other. The only flanking going on tends to be the Rogue flanking with me (the Fighter) because my mark can keep the enemy from attacking him. So, there's really no advantage here for our party, since he basically has to be in melee in order for someone in our pary to be flanking.
(This is basically a real party example of what DracoSuave was saying. It's not that we don't value CA and flanking, just that typically it's better for the Melee Cleric and I to be adjacent rather than flanking a single enemy because of our party makeup.)