It does matter when the future games that are going to be released for those other systems are going to need to be dumbed down to run on the Wii U. It's going to be the Wii all over, you remember all the crappy ports the Wii got because of it's weaker hardware, the same thing will happen with the Wii U. In my opinion, Nintendo don't stand a chance unless they release a console that matches one of their rival's consoles, if not both of them, hardware wise.
Look at it this way, if you were going to buy a gaming computer, which would offer you the best experience, the high end computer or the low end computer? Well, the same can be said for gaming consoles too, if you were going to buy a gaming console, which would offer you the best experience, the high end console or the low end console? Most of the true gamers out there don't want sub-par hardware to game on.
Actually, the Wii U is strong enough computationally that ports should not suffer that much if developers take the time to do them correctly. The Wii U CPU is, in single threaded performance, in the same ballpark (perhaps even faster) than the Jaguar CPUs. The difference in GPU speeds between the Xbox 1 and Wii U is similar to the difference in speed between the Xbox 1 and PS4.
By contrast mean, the Xbox 360 GPU was putting out something like >10 times the single preceision FLOPS as the Wii. That was a dramatic difference, plus it had a much faster, multicore processor. Both the PS3 and Xbox 360 were poorly engineered attempts to fit as much raw power in a small case as possible, resulting in a thermally disastrous design.
After learning from their mistakes, Sony and Microsoft designed the Xbox 1 and PS4 more like the Wii, with a thermally efficient, low power design. They are not making the kind of power leaps that they did in the last generation. The Wii GPU is noticeably slower, but not dramatically slow. Between the Wii U and the PS4 there is about a (5 x) difference in GPU single precision FLOPS. Between the PS4 and a high end PC GPU set, within a few years (when developers really start taking advantage of the new consoles' specs) there is going to be a 50-100 fold difference.
Most of the actual improvements will be in the software. Ports to PC's that are 100 times as fast as the PS4 will not look 100 times better. They will look more similar than different. Same with ports to the Wii U. You'll be able to notice the difference, especially between a PC port running in 4K and a Wii U port, but it will not likely be that dramatic if developers do their jobs right. It will be nothing like hte awful Call of Duty ports for the Wii.