I am kinda surprised by the number of people who think a 2-hour session would be a "waste of time". I am not saying that's an illegitimate position to have, just for me it's a surprising one.
Maybe it has to do with assumptions about the game:
1) Are some of these claims based on an assumption that the game involves an extensive run-up to get started?
IE Are you expecting time to generate a character, or for extensive DM prep while the players are present, or people talking a lot before the game starts, or a re-cap of recent events in the game, or buying and selling stuff in town before the adventure begins, or other stuff like that?
2) Are some of these claims based on the quantity of time it takes for the DM to prep before the players arrive?
3) Are some of these claims based on the idea that random encounters cannot further a longer cohesive campaign?
For me, I can (and did) sit down and immediately begin playing with no run-up time. I can (and did) do it with no prep time. And I can (and did) do it as a part of a longer cohesive campaign.
I've done this both in the context of a dungeon adventure, and a wilderness one. For example, if the players are travelling to a distant location as part of the larger campaign to help achieve some goal, we've had short two hour sessions where we all just sit down and immediately roll on random charts for what they find in the next hex they are travelling through. That could be a friendly encounter with a Dryad, a hostile encounter with a den of owlbears, a neutral encounter with a fellow band of adventurers on the road, whatever.
And these can be linked together as the party travels towards their destination as a series of encounters during that travel. No run-up time, no prep time, part of a larger campaign, and lots of fun.
It surprises me, because I thought most people played this way. Heck, I view the novels and other fantasy material D&D was based on as including this concept in them. For example, Bilbo and the dwarves encounter three trolls on their way to Rivendale in The Hobbit as, from my perspective, a simple random roll to see what is in the next hex in that wilderness trek.