There have been lots of series that have transferred systems. Final Fantasy, Metal Gear, Mega Man, Resident Evil. So I don't think the 'you have to play the previous ones' holds much water. Maybe for some of the convoluted story, but that is what reboots are for. What if he makes NMH3 not need too much knowledge of the previous games?
I'll buy it even if I know the third is coming to another system. Firstly, it could come to both systems. Secondly, NMHs will be a great game on it's own (I'm assuming).
Time will tell. I say everyone buy it (assuming it is really good) and show what kind of sales it can do on Wii. Then if it flops on that other system, they will bring it back to us (like with RE4 and other games that were eventually ported).