Impermanence actually offers the opportunity to try out something new without committing.
Especially for those who like really long campaigns with low-lethality. If you are going to play that Barbarian PC for 2 years continuously, why would you want to pass on the chance of trying out something different for one evening? It could be a good chance to test out some mechanics you haven't tried before, such as spellcasting or sneak attack. Maybe you like the "preview" and decide that later in another campaign you will play a fully-fledged Wizard or Rogue.
That's true... but also at a right angle to the point.
The point of contention is not whether there is anything to be gained for a short play of D&D in and of itself... but whether the defined specifications of the scenario that the OP posted would be a worthwhile time to do that. My personal feeling is 'No'.
I play D&D biweekly currently (and it was weekly for several years.) Anything D&D-related that I want or need to do can be accomplished within that biweekly window, or at the various locations I might go where D&D is specifically being presented (in whatever format it is). Conventions for example-- I've DMd D&D at PAX East for several years, and I know going in (and indeed have scheduled it) that I'm playing small con games of D&D. I'm good with that.
But in a situation where D&D is not the defined event of the evening... like the scenario the OP originally put forth... for me personally I would find pulling out
more D&D to be a waste of an opportunity that would be better served playing something else that I do not ordinarily do. I haven't played Fiasco is over a year. I'd LOVE to play Fiasco again. So if we had the scenario as posted where a bunch of friends are over and we decide to play a game (any game)... playing D&D would be a loss of opportunity.
I imagine the big hang up right now is all of us using the phrase "waste of time". Some of us are using it metaphorically and some of us are using it literally. I don't think any of us really think playing D&D in any situation is a real crippling loss of 2 hours of our lives that doing ANYTHING else would be preferable. That's just silly. We're using "waste of time" metaphorically. It's more that we consider those two hours a loss of opportunity to play or do something else that might be even more enjoyable. But other folks who are countering us are treating our use of the phrase "waste of time" as though we think playing D&D is the absolutely worst thing we could do-- like we'd rather stand in line at the Department of Motor of Vehicles for two hours to renew our driver's licenses than play a one-off game of D&D. So they keep offering up reasons why playing D&D is better than that.
Which we know is true. And which is why we'd prefer you not take us literally by our use of the phrase "waste of time", but take it in the spirit of how its being used. Which really should be rather clear.