It's widely known that the three class groupings are misleading- heavies and demos make excellent attackers, whilst accurate scouts make devestating defenders. Melee snipers and battle medics can even be classified as attack classes, even though they're classes as support (so get on supporting!)
The engineer grouping is, I think, the most misleading. Two games today helped solidify my opinion.
game 1- 9 on 9, Goldrush, defence. We had a 5 strong engineer team. Each build up sentries by point 1. We were steamrolled by ubered demomen and spy combinations.
game 2- 9 on 9, 2Fort, highlander. As a purely defensive engineer, I ended the game with a kill

eath ratio of 5 to 23. The last 3 kills came from me turning into an offensive engineer, putting SG's in the water.
The engineer is, in my opinion, a support class. My reasons?
*3 out of 4 of his deployables are geared towards helping his team indirectly (the dispenser and teleport)
*His lacklustre health and basic stock weaponry prevent him from engaging in head on combat and coming out alive. Any engineer can plunge into the fray, but very few will come back in anything other than a bodybag.
*
The sentry is a weapon of suprise. Yes, it will shred scouts and any unaware class, but people wise up. This brings me neatly to my next point-
*
The sentry is both dumb and immobile. It locks on, it fires at whatever enemy's closest to it. If you dodge, get far enough away, circle strafe it or sap it whilst it's alone, it's toast. It can't intelligently hit classes such as the scout and the demoman who doge in and out of cover, lock onto good circle strafing pyros and can't move to attack ranged rockets or sniper bullets.
*Uber vulnerabiltiy- an uber demo/ heavy/ pyro/ soldier is heaving your way- o shi-. You run. A sentry fires and is blown up by any competent uberee, so long as they get close.
A counter arguement- but Numenor, I've had 10 (or 100, or 1000, or whatever) kills with a sentry and we've won the game! Not to mention all the scouts I've stopped!
True, an engineer excels at stopping rushes- but in my mind that's the only real defence aspect of him. As for the 10 kills- any engineer that does well is (most of the time) on a good team. An example is stage 3, point 1, Dustbowl. Setting up by the stairs opposite the driop down window from spawn, an engineer will either rack up kills if his team helps kill the enemy medics and push the enemy back. In a team with poor team support, the enemy will have little to distract them from their target- to blow up the engineer's vulnerable sentries.
This isn't trying to improve the engineer- nothing is wrong with the engineer, and can often be the only thing that stands between a successful enemy push and a failed one. But to everyone out there who believes their CP/ Intellignece is safe 'cos it's guarded by an engineer, think again- he won't last forever.