I guess I've been lucky and not ahd a bad crash, but I know that if things did go wrong (which they often nearly do) when I'm riding the resulting injuries could be very bad.
Of all my injuries -
A stray branch that ounctured my shin would have been stopped by a shin pad, however this would only have helped if i wore them all the time rather than 'padding up' as it was a normal trail.
Banged my knee on stem several times, once on a rock. Never drew blood but it did hurt. See above re padding up.
Crashed and flew superman style into an uphill slope bending my back back the wrong way - thankfully nothing more than a sore back but the relevant precautions would mean a spine protector (do they work in that way?) on an xc ride.
Sprained wrist on a regular xc ride - wrist protectors may have helped.
Rocks into shins - it happens but thankfully they tend to go into the downtube or don't hurt too much.
Like I say I may have been lucky, but as far as I can see armour only protects the obvious pointy out bits, but you're just as likely to hurt the bits that aren't protected as those that are.
I broke my leg playing football along with 1000s of others - what armour should I wear for football?
Isn't danger part of the sport?
Its all your own choice but for me the expense / uncomfort / hassle isn't necessary. I guess a parallel could be rock climbing - some don't wear helmets, some don't use ropes, some use matts when bouldering...