In Yosemite in winter, if you call yourself a backpacker it probably means that you are going to one of the low elevation trails such as at Hetch Hetchy. There, the snow doesn't get terribly deep and you might make it without skis or snowshoes. However, for most of the park, you will be up into the deep snow country pretty quickly, and you will need either cross country skis or snowshoes. If you travel out the Glacier Point Road, it is common to see a ski track on the right and a snowshoe track on the left, so you can tell where everybody is heading. If you are looking to just spend a night or two out in the snow and then return, then that is the way. OTOH, some people are really trying to get from Point A to Point B in the park, and there the skis and snowshoes become vital, and you actually spend less time camping. As an example, a standard trip is to ski across the park from east to west. There is a free skiers bunkhouse at Tuolumne Meadows. First come first served.