arezey pretty much mentioned the round-about reason for the invasions - they are merely for decoration - not just for corpses (though that is intended to be part of it), but for aesthetics to the map itself anyway. Consider them interactive decorations, if you will. They also were, in a way, an experiment, to see how well they would be received.
The demons are not intended to have a significant impact on gameplay, so that is why there are so few of them and why they are so weak. A moderately skilled player who knows what they're doing can make short work of a baron of hell without taking any damage whatsoever.
Yeah, they are a nice distraction though, and if you're really hurting for experience they can provide a minor boost, depending on how many you kill. And since the game behind this wad currently is, well, Doom, I figured it'd be nice to add a touch of "Doom" into the wad, and hence, this map, and the hellspawn invasions.
This is also why Cyberdemons or chaingunners won't be added to the map anytime soon.
