As its name suggests, the Wilderness Act of 1964 has been in effect for fifty years, long enough that most Americans have grown up with it.
From a campfire-argument point of view, I could say that our culture is weird if we have to draw lines around a small portion of the country — only 2.7 percent of the Lower 48 — and say, "In these places, natural processes are more important than the human ego."
In other words, plants and animals come ahead of human exploitation, whether that be for economic or recreational reasons.
(Like Gary Snyder, I define "natural" as those self-organizing processes not under the ego's control — including most of what your body is doing right now.)
On the ground, the "wilderness" designation usually means no engines, no wheels. If you want to do in, you walk, ride a horse (or other equine), paddle, or float. Some of these can even be done by people with disabilities!
Come now two senators from Utah, Orrin Hatch and Mike Lee, who want to allow bicycles in wilderness areas.
Since I really doubt that either one of them lives for mountain biking, I suspect that this is just a thinly disguised attack on the very idea of designated wilderness. They don't care about bicyclists as such, they are thinking about oil wells etc.
As the "camel's nose under the tent," mountain bikers work pretty well, better than ATV riders, for example. After all, they are "using the quads God gave them," as a certain anti-ATV bumper stick says.
But they still don't belong in designated wilderness areas, not under the spirit of the Wilderness Act, which has pretty well proved its worth in fifty years.
Yes, bikes are quiet(er) than motor vehicles, but as they rush over the trail (go to get cool vid on that helmet-mounted GoPro camera, right?), they are still a disturbance.
Let's keep Wilderness Areas as they are, places where the needs of plants and wildlife come first. Sure, we can go there with respect, but our desires to put knobby tires everywhere in the name of recreation can be limited in these small slices of America.
If you think that mountain bikes are cuddly and harmless, you can make your case — but then you are opening the door for the next mechanical intrusion. And the next. And the next.