Without randomization of the tiles, the replayability of a given quest, or campaign, deteriorates quickly.
Sure it is exciting the very FIRST time you play it, but that excitement of the unknown diminishes exponentially the more you play it. Further, by randomizing the tiles from your ENTIRE collection, it allows for some very interesting dungeon crawls, especially if you actually own everything of D2e, as I do. It would even make good business sense (for FFG), as this will tend to drive additional sales of their expansions. Quite frankly, we are already seeing this, via anecdotes, of this exact thing happening.
If the dungeon tiles are not random, it becomes too easy to know that if you setup your heroes in such and such a position, it will be exactly X spaces to this critical area, etc. This is a very simple example, but you can imagine other scenarios that if you know the precise layout of the dungeon ahead of time, it can impact the success factors for the heroes, and thus limit the enjoyment of game in the long run.
All of the above would be a moot point if content for the game came out every week. I just don't think this will be the case. I think we will be very fortunate to get 1 or 2 complete campaigns every year. Another reason why I suspect people are asking for the ability of linking in the Quest Vault designs into the game, as this would provide a far larger content world.
Edited by any2cards