Well since you want to talk about this in the context of DB, might as well.
Because the map's essentially an extremely long choke point, engies wreck face with sentries. On defense anyway. The defending team moves back anyway, so forward sentry positions on offense kinda don't matter. Teles are nice on offense, but the time it takes to get from spawn to the point isn't that much so it's negligible.
But medics can uber people and destroy the sentry as well as continue with the carnage. Uber's also the best option against another uber, so good defensive there. Kritz just destroys people as long as there isn't a sentry. (which isn't likely on DB, but makes kritz an excellent defensive option) Ubers are also great for last min. saves when the demo spam on DB's getting to you or if you accidentally run into a sentry.
So in terms of DB, engies are excellent on defense, but medics can do offense and defense. Therefore medic's a better choice. (not to say that engie isn't important either)
Medics:
-Only heals heavies
-Only ubers when his own life is in trouble
-Needle gun is useless except on spys and scouts
-Crossbow is great, but only with people who use lucksman.
Engie:
-Can create hell with gunslinger/frontier justice
-Can be perfect offensive engie with jag and ranger
-All shotguns are OP, especially Pomson
-Can be awesome defence with Widowmaker/Short Circuit/Eureka
-Can heal ALL teammates at key points in maps.
-Can use teleporter for good or evil
-Distracts spies from killing real targets
-Dances
Are we still talking about this?
Medic:
1. You're implying that Medics, by default game mechanic, can not heal anyone else. Which is absolutely incorrect.
2. Medics are supposed to Uber when they have seconds left to live. Pop it, don't drop it.
3. You're assuming that needle guns and the crossbow are the only defense that Medics have when in danger. Medic melee is pretty decent esp. considering he's the 2nd fastest class in the game. Needle guns serve as great deterrent tools anyway. They're not necessarily for killing people.
Engie:
1. You're assuming Engies are automatically good as long as they have the right loadout.
2. Shotguns have absolutely terrible range aside from the Pomson and require more than decent aim.
3. Again, you're implying that only Engies can heal everyone. Medic heals faster and can actually heal
anywhere.
4. You're implying that Engies aren't real threats at all and that it's a waste of time for Spies to be going after them. Which at the same time implies that Medics are more important so your argument there contradicts itself.
Like Mr. Monday pointed out, your argument is full of inconsistencies, fallacies, assumptions, and implications. It is therefore null and void.
That and your attitude is not fitting of a debate/discussion.