The problem of the dungeon being constrained to the grid comes about BECAUSE of the grid system (IMHO).

Example:

In a game without miniature representation, it is easy enough to just draw something out and say: "This is roughly what it looks like"

But as soon as you put any sort of empirical measurement into the system it falls apart.

My character can move X amount on my turn, but your character moves Y on your turn, and the Monster/Villain moves Z on it's turn, can attack further, but your character can shoot a certain distance. How are these measured on the battlefield? Thus comes the grid (be it triangles, Octogons, Hex, Circles or whatever).

I see a grid system of either squares/Hexes as the most simple because I can 'eyeball' it on both the 'Big Table' with the miniatures set up and the adventure map. By utilising a grid (lets say Square), I can see in the adventure the room is 6 squares by 5 squares, with a 5 square long 1 square wide set of stairs entering the room on one side.

With the grid I can ensure the battleboard is identical to the adventure.

Now, as an aside, at GenCon last year for the BIG D&D 4e presentation, they had a temple set up in the hall with a map on the floor set up in the 5' squares the game is based on. I was talking with the then head of Organized Play, Ian Richards. I stated that in reality, saying that I occupy a 5'x5' square is a lot of territory to stand in, and in reality I can see just how 'ridiculous' it looked to say in a 10'x10' room only 4 people can fight.

But that is reality, and the game mechanics need to be addressed for an internal logic. Yes, the grid is unrealistic. Yes, dungeons built with rooms in variations of 5' increments is unrealistic. I know this, but the game needs it, to make sense. Otherwise, you start having incidents (regardless of how level headed everyone is) of people assuming they are here but another assumes they are there. Incidents of one combat 10 people involved in a melee in that 10x10 room and another incident where it's deemed you can't.

This is known as the game's internal logic, and is an example of what I have a problem with in the Spider-Man movies. In the comic strip he had web-shooters he loaded with compressed chemicals to make his webs. They felt that in the movies it would be a little hard for the 'fans' to handle is ole Petey could make a Chemical a 3M company cannot (that being synthetic spider silk), so they decided he produced it himself. WHAT A MINUTE! He's putting out each time he shoots his webbing the same amount of fluids and proteins as a normal male does each time he ejaculates (sorry, but it needed to be said). Now, how the hell does he keep his body full of nutirents and wtare etc without becoming extremely dehydrated after a simple fight. The internal logic has fallen apart.

SO, again, the D&D game system demands that buildings/dungeons be built along the 5' grid system as that is the internal logic of the game system. It also means no 'wasted space' as the game states a M sized creature can fight within a space smaller that 5' but is deemed 'squeezing' into the space and takes a penalty (see my point above about the reality of the 5x5 grid).