In a mirrored map every engy on your team is one less player that can help you win. Engies only delay losing.
This is true in some instances, but not true in others. It's definitely true in 6-6 play, but definitely not true in most 12-12 play. A good engineer will place his sentry near the frontlines, which basically holds that line on the map plus acts as another gun. Whenever his team pushes the line of control forward, he needs to move his sentry forward as well. In addition he'll have a teleporter going forward as well, which allows his team to use larger slow classes against smaller classes more frequently.
True that in many cases it's better to just have another demoman, medic, etc. However making a blanket statement is ignorant. TF2 is a game of situations. There are situations for almost everything, and having an engineer on a 5CP map is one of those situational things.
In addition, I guarantee that if you put two equally skilled teams against each other and you have an excellent engineer on one side, his team will win. I can't count the number of times on 5CP maps that I've worked my way past the battle and gotten a teleporter up behind the enemy's next CP, and then followed it up with a surprise sentry. That almost always guarantees your team will cap CP4 and be able to push CP5 with little effort.
On payload maps it's just as effective. Mangy has seen demos of me getting teleporters behind the enemy and using sentries and my shotgun to slow the defenders. Even if they destroy my stuff easily, that's a little while that my team has to push the cart without any opposition. I've won Badwater on BLU nearly singlehandedly a number of times with excellent teleporters and surprise sentries. Good engineers can do that.
I'd suggest before making more blanket comments about the game that you play once with an excellent engineer. No, good engineers won't need locations set up specifically for them. However, maps without good sentry locations tend to play VERY fast with scouts quickcapping many points. In 6-6 that's fine, but in pubs it tends to get a bit boring. The same can be said for making multiple excellent sentry locations, like on Hydro. The key is finding a middle ground with multiple good locations spread around.