ride with a good backpack and a bladder full of water. That will protect your back as much as any XC backpack
The problem with protection on pedaly ride is that it make you hot. There for you will dehydrate faster, drop your attention level, make more mistake and fall more.
It is all a balance between protection and weight/heat.
If I only look at myself, the better I become the more stunt I pulled (jump/drop/etc..) the less protection I wear
If I have an uplift, then it all goes on. If I have to pedal, enduro open face helmet, knee pads, glove and a backpack with 2 to 3 litters of water is all I am happy to put on...