Maybe keep it....
Well, I don't know about the other people in here, but I think it's worth the reg-fee anyways. Think about it like this; DotMobi's are domains/websites designed specifically for Mobile phones, right? Right. So the shorter the domain, the less times the Mobile Phone User has to hit 4 666 666 4 555 33 (spells Google). Get my drift? Hang on to it for a while, and try to develop it. See what may come of it.
If it doesn't work out, just remember that everyone in here, including myself, has bought a few sucky domains because "I" thought it was great while no one else did. Good luck bro.
Tory