I'm not sure who those "many people" are but it's crap. Don't worry about coming unclipped if you're using the Shimano SPD system and the SH51 cleats. The basic M520 pedals come with the right cleats and are a great way to start.

I've pulled out of SPDs while MTBing, but every time it was because I was actually off-axis or doing something stupid like trying to make the bike move sideways while bunny-hopping. In all skidding scenarios you'll be firmly pushing/pulling with minimal side-to-side movement.

Bottom line: Go for it! If you're brakeless you'll really appreciate the solid connection and stiffer soles of clipless shoes. Even a cheapo $70 clearance pair of Shimano shoes will be lightyears ahead of sneakers and straps.