"Dangerous" is relative. In most cases, you won't sustain much harm by doing so, although the items in your backpack might get crushed. I have had it explained to me before by my former teachers (jujitsu and Bujinkan) that this is one of the reasons some of the defenses involve turning a flip to escape from a hold rather than a roll, because it prevents you from rolling over your sword. At that, a breakfall may also be a better choice in some situations. Learning multiple options lets you react appropriately for the situation.

I suppose it might depend on the content of your backpack too. Breakables, sharps, and objects with lumpy corners could dig into your spine or organs close to the surface. *shrug* It's probably about the same risk that you'd face if the contents of your backpack were on the ground and you rolled over them.