They should bother because the more effort and games they put into a console library, the more potential consumers it opens them up to, and that can only be a good thing. If only third parties put the same effort and care into Nintendo console games as they put towards HD consoles, the sales will (eventually) come just like they do to the PS360. After all, why wouldn't they? What makes the games worse and less appealing on Nintendo consoles than on the PS360? Esepcially now that Nintendo has a console that is close to them in power. Not to mention there is the additional selling point and distinguishing factor of the gamepad.
There is really no excuse left for third parties NOT to invest in Nintendo consoles anymore, other than the lame fall back excuse of "only Nintendo games sell on Nintendo consoles, whaaaaaaaa!" BS. Make your games good enough and you will earn the money of Nintendo gamers. That excuse simply tells me that the developers are not good enough, and really shows the decline of quality of this industry as a whole.
It really comes down to the chicken and the egg argument. Most will say that devs don't support Nintendo consoles because gamers don't support them, but I contend it is the other way around.