Component video isn't useful just because it's capable of carrying an HD signal.
It provides a nice boost in visual quality even at 480i hooked up to a standard definition CRT. That's why the majority of standard definition CRT's produced over the last decade or so of production included a component connection, even including budget ~13" sets over the last few years.
It's split into three signals so there is less signal degradation and the television is able to recieve the most accurate representation of the video being sent to it compared to composite or s-video. I'll spare the technical details for someone more tech savvy to explain to you why it's to your advantage to use, but suffice it to say that it will give the picture quality a nice boost at any non HD resolution. So it's very worthwhile using on the Wii despite it's lack of HD.
It's also the only way to get 480p output from the Wii, which will further increase the picture quality if you have an EDTV or an HDTV. And it also can help your experience on an HDTV since the television doesn't have to deinterlace the signal (Most of the time, there is the occasional quality Wii game that is 480i only) which can be problematic in terms of picture quality and input lag if the scaling chip in your set isn't very capable.
It even gives something like an Atari 2600 a nice boost in visual quality after modifying it to be able to output component video. So it's certainly worthwhile on the Wii and very cheap to do these days (You should be able to locate a decent 3rd party component cable for $10 without any trouble), unlike on the GameCube with it's expensive, online exclusive component cables.