Why would i want a port of a game i already play and own?
because it would be portable. Thats why.
Why is portability such a big deal? That alone is not a selling point. If there were extras and the controls were completely reworked to to take full advantage of the 3DS's setup, then yeah, it probably would be. However, simply porting a game that someone already owns without any sort of enhancements to a system where the controls would suffer isn't grounds for buying it.
The ability to play the game during a vehicle ride, school/work breaks, etc. isn't reason enough; especially when one could play the original version from the comfort of their own home on a console that is still relevant(for a while anyway). The only types of games that would be worth porting, unaltered are games for outdated systems that are hard to find and that's already being done.
What's the price? If it was five, maybe ten dollars, I'd be fine with a chance to play one of my favorite games whenever I like, seeing how I can't easily access my Wii/Gamecube at the moment. If it was more, I'd be willing to wait until I did have easier access.