First off I don't have Civ V and don't plan on getting it, but I never felt a problem with unlimited units per tile either in the game or considering 'realism'. I've seen people playing Civ V and I honestly can say I like Civ IV better (in many ways Civ II and III still compete in my head for my favorite, but my favorite mod is in Civ IV).
I feel like the attempt with 1UPT was to create a tactical feel to the game, but the scope of the game is strategic by default and I don't feel it meshes well. A few tiles in Civ IV represents what would've been nearly a full map in Pazner General.
I'm glad a lot of people like it because I want the Civ franchise to be here for the long haul; it's just not my thing.
The amount of space that a single tile represents 'in real life' is vast and it wouldn't be impossible to fit hundreds of thousands of people in an area the size of downtown Chicago; 43,000+ occupied a stretch of beach in WW2. There are numerous examples in history when multitudes of people fit in a small space, if just for a short period of time.
I do think there should've been some kind of limit put in place, like depending on the terrain a certain number of units would've been allowed: grass/plains=15 units, forrest/jungle/hills=5-10 units, mountains=1-2 units. If this was combined with the ability to damage multiple units in the stack with a bombardment then I would think it would've been a better route to go other than 1UPT.
I guess I'm just going to be waiting for Civ VI then.