So I only read through a couple pages of posts and I think I get the general arguments being thrown around.
First of all, I can't believe we have people arguing about the validity of a BASIC ELEMENT of the entire game! TF2 was DESIGNED for each class, with each class in mind, balanced for each class. How can anyone possibly consider questioning the role of an Engineer, it's as fundamental to the game as the other 8 classes.
Next I have to completely disagree with Mar on all points. I'm not going to go through and counter his points as it's already been done, but again I bring up the point that the game was designed with every class in mind, Engie included, and as such it is every mappers obligation to make sure he is catered to, whether you like it or not. This isn't Quake or Unreal Tournament my friends, let's not forget that.
Being that the game is team-focused, I will also remind you that no class, Engie or anyone else, should have the upper hand against any number of individual classes. Yes, there is a very clear difference between a good Engie and a bad one, and the margin can be extremely wide, but all Engineers have one thing in common: without their teammates' support, they will fail, Sentry or not.
Engineers are generally not gun-hugging campers, they are providing a service to their team, playing a role. Sentries more than any other aspect, force a team to work together to take it down and control an area. The way I see it, short of being able to set up 4 Sentries that are impossible to beat without 4 ubers, ANY Sentry emplacement, no matter how strong or weak, is good for the game. No Sentry, or even 2 Sentries, are impossible to take down, it just requires more teamwork and patience to build up an Uber or two to handle them. It's amazing how many times teams throw themselves at Sentries individually until the last 2 minutes of a game, THEN decide to charge an Uber and end up winning.
With this in mind, every point or main game-area (hotspots) should have a viable place to put up a Sentry. This means an area that is not too cramped that a Demo can place Stickies through a doorway without harm, but not open enough that a Soldier can easily pummel it with Rockets. This DOES NOT MEAN making a totally covered spot though, every spot should have it's weaknesses. While a good Sentry position should force an attacking team to work together to take it down, that same position should require the aid of a defending team to keep it up!
A tip while mapping (which I stole from the interview with the VALVe mapper, forgetting his name at the moment), place the Sentry model Hammer to designate Sentry positions and set the Max Fade Distance to 1100 units. Then selecting the Sentry model will give you a grid-radius that is about the distance a Sentry can recognize enemies.