The way the game is played now has of course changed drastically from its launch day incarnation. Back them, if you wanted to run an dungeon and you didn't have friends who wanted to go (perhaps they weren't on, perhaps they weren't interested, perhaps they'd done Dire Maul seventy seven times and didn't want to go back because that Quel'Serrar book just wouldn't drop) your only resort was to work your contacts in other guilds and have friends stand around in major cities hitting the various channels looking for people. I tanked a lot of dungeons after a desperate group of strangers sent me a whisper seeing that I was online - it got to the point that I felt obligated to tank those dungeons, because it felt like no one else was doing it, and that made me resent it.
Anyone who remembers those days fondly, well, I wonder what realm you were on. On Azjol-Nerub and later Norgannon, the days before the advent of Dungeon Finder were an entirely different world, and one I often found keenly frustrating. I won't even talk about how certain classes were almost unwelcome at certain times - DPS warriors were never popular in dungeons before LFD because they have no CC, and back then CC was a huge deal. It was rogues and mages or GTFO, unless enough hours had gone by to make a group desperate enough to take anyone. Yes, I said hours. As in multiple. As in, you could spend six hours standing in Shatt waiting for a Shadow Labs group.
I mention this not merely to confirm the stereotype that I am an ancient thing that should be sitting on a grim sepulchral porch somewhere on a rocking chair made of skulls and stained with blood - for one thing, who even makes those? - but also to point out that when the dungeon finder came to be, it changed the way we run dungeons and the game has evolved to accommodate it. And that means that yes, we need systems like the deserter debuff. We need systems that replace the pressure we once felt after hours of work to get that dungeon done, wipes or no wipes. When some players could theoretically drop group every single time anything happened that they didn't like and immediately get a new group, there has to be a means to prevent it because the system would collapse.
World of Warcraft is a multi-player game, and as much as it has options for single player achievement (Garrisons come to mind as almost exclusively single player content) when you run a dungeon or a raid via the game's built-in matchmaking tools, you're signing up for something that requires a time commitment. Fifteen minutes before you can bail on that commitment without consequences doesn't seem very onerous to me - you can spend that fifteen minutes in the dungeon, or spend it cooling your heels outside the dungeon. But either way, it's not too much to expect you to make that attempt.
Now, we could argue that the deserter debuff is poorly implemented. That it punishes indiscriminately, so if you're in a group with (as an example) three guilded players who keep making the tank miserable so that they drop group, you have to sit there and soak that up for fifteen minutes before you can drop. And that's fair. It would be nice if the report feature was more robust, and the debuff smarter. But the idea that it shouldn't exist at all and we should be able to drop group immediately with no consequences would be destructive to the system. Yes, you can leave group whenever you want - and if you leave too soon, you can take a small break.