Yeah, the reason for choosing uClinux in the first place was that it doesn't need an MMU. That means a badly-behaved program can mess up other programs, or even crash the whole system, but that's no different than under System 6 or 7.

The other downside of not having an MMU is that physical memory can become fragmented over time. You might have more than 500K of free RAM, but can't satisfy a 500K allocation request, because the free RAM is scattered across lots of smaller chunks. As far as I know System 6 and 7 suffer from the same problem, though, so it's not the end of the world.