I'm just trying to understand this because it's like saying buying a Megamillions ticket, but my ticket is also good for Powerball. Sounds too good to be true, but I guess a 2% bonus isn't really much anyway, so heh...
I'm not sure about 2-3%. Here are some stats from my account on BTCGuild:
Bitcoins mined: 89.79664017
Namecoins mined: 208.32637317
That's about 2.31 namecoins for every bitcoin.
From https://exchange.bitparking.com/main, namecoins are currently worth 0.00968888 bitcoins.
That means for the 89.80 bitcoins I mined, I got a bonus 0.87 bitcoins. Not including fees to sell the namecoins (which I haven't yet).
Work it out to see how long it'll take you get 1 free bitcoin. That's about 1% bonus, rounded up.
Not worth the 5% fee IMHO.
(I read elsewhere you can run the pool software on your PC if you really want namecoins.)