I can think of a few reasons... A new approach some merchants have taken is to accept only Bitcoin and thus eliminate payment processing fees. This leads to prices lower than their competitors can reach (stores such as Bitcoinstore.com and Bitcoinin.com). While the 2-3% may not matter much on small purchases, larger orders certainly will.
Another aspect is the peer to peer nature of Bitcoin. With Bitcoin you have the ability to send money to any other Bitcoiner, anywhere, instantly. That simply isn't present with traditional currencies. Now there are some inherent difficulties there. For instance, if I need to send money to my family on the other side of the country I can do it instantly, but they still need to be able to translate Bitcoin to whatever good/service they need. Currently that may require translating it into their local currency by some means, as not all that many local/physical merchants accept Bitcoin yet.
Another aspect is the peer to peer nature of Bitcoin. With Bitcoin you have the ability to send money to any other Bitcoiner, anywhere, instantly. That simply isn't present with traditional currencies. Now there are some inherent difficulties there. For instance, if I need to send money to my family on the other side of the country I can do it instantly, but they still need to be able to translate Bitcoin to whatever good/service they need. Currently that may require translating it into their local currency by some means, as not all that many local/physical merchants accept Bitcoin yet.