When mountain bikes first came out, we had very little choice with what setup to use. The hot setup was a cut and rejoined Mavic rim (MA40, about 21mm wide if I recall) running 2.2" to 2.5" tires at 15-30 psi (no suspension forks). I ran 2.35" tires on similar rims for the longest time with no problems.
The problem is when you put a tire narrower than the rim (or close to the same size). That's when you start having a lot of pinch flats.
I'm not sure if things are different with current mtb's but I never had any difficulty mounting or dismounting tires with the setup. I struggle more with the road tires for sure.
hope this helps,
cdr