Lugging it from the spawn to the front line leaves you susceptible to Spies who are waiting for just that sort of thing, to come and stab you in the back while you're holding a Level 3 building.
You usually take the building through a teleporter, which is much safer.
Some maps can leave engineers starved for metal. Badlands in general, or on some maps multiple engineers that have to compete for a limited metal supply to get buildings up in a short amount of time. For instance, quickly getting built on the roof of Badwater. If an engineer already has a tele there, they can simply bring a dispenser/sentry through it.
Douchebag teammates taking your metal can be a problem. If it's in setup they should really be doing what comp players do and have some non-medics suicide so the engi can use their weapons for metal, but otherwise they should have the common sense to wait for the dispenser to be built. Them taking the metal I was going to use to construct it isn't helping any.
Personally, I build first where I need to be and let any engineers that are going to be carrying a teleporter use my tele. If it is in setup people that aren't playing engineer can handle tele and sentry upgrades. That's one reason why I like the Eureka Effect...I don't drag buildings everywhere, I just make slight position adjustments as needed so the loss of it isn't a big deal. You can head to the frontlines, drop a teleporter and dispenser, teleport back to spawn to upgrade the tele and/or get metal then head back.
