I'm a frequent parishoner at the Church of Nature.
I firmly believe that motor vehicles don't belong in public-land wilderness. The only exception would be emergency vehicles.
If you want to ride a dirtbike, go to private land where dirtbikes are permitted. If you want to ride a snowmobile, 4-wheeler, or 4wd truck, same thing.
Motor vehicles cause too much damage for the tradeoff of one person's mere enjoyment and testosterone-induced thrills. This includes erosion and subsequent degraded water quality, noise pollution and its effects on wildlife, exhaust, and in the case of roads, fragmentation wildlife corridors resulting in altered ecosystems.
If you want to reach a remote lake, map it, plan the long hike, and appreciate it more because you actually had to work and "commune with nature" to reach it. Communing with nature isn't just about reaching your destination, it's about integrating yourself over a meaningful period of time. Communing with nature does not mean "conquering it" or "taming it" either.
As for the Nature Conservancy, my preference would be for allowance of primitive access (no vehicles), with smaller set-asides near populated areas for more formal nature trails, most being ADA accessible. They may find the need to declare certain sensitive wildlife habitats or watersheds off-limits, but even in these cases an appointment with a guide or permission for one-time-use access is frequently negotiated.