While I think group parenting is fantastic, and a poly family who wanted to have kids where even the non-biological parents are treated equal is amazing - I would want to know the paternity.
What about genetic issues? What about family medical history to assess risk? What if the kid becomes curious about knowing where ancestors came from or something like that?
I don't know.. I would want to know. My family has a lot of crap that comes with the genes, though, so I wouldn't want to chance having unidentified added risk just to make sure one man doesn't get jealous of the other because his sperm was at the right place at the right time. You can legally arrange for co-parenting rights (which I also think is the smart way to go), and really, I agree with NYC - it's not the genes that make the parent, it's the love.