Demons themselves drop rune materials, for example terror demons drop spirit essences. Not just with rifts, demons have a set chance to drop certain materials. A rage demon just hanging around the Exalted Plains is just as likely to drop a fire essence as one from a rift, and dispelling a rage demon at a rift makes you miss any chance of it dropping anything.
Yes, when alone or in non-rift mobs, demons drop mats. But if spawned from rifts, I've never gotten mats from a direct demon drop, only research items. I suppose this is a game code rule to insure you can't double up on rune mats, since the rift always drops a rune mat, and more than one with a lot of other goodies in JoH.
And regarding the OP, it's another game balancing attempt that you can't dispel a demon you can identify. That would be pretty cheesy, right? Either roll the dice and close the door before you know what's coming through (good analogy, there), or identify the thing coming through once its too late to shut the door.
Personally, I dispel as many as I can. At this point, after multiple PTs, fighting demons to close a rift is just tedious. Except for the Pride and Fear demons, those are fun.