^ This.
I know I already said this, but I want to reiterate: When BLU Engineers could still build Dispensers, I saw them use it for blocking alternative entryways to Armory. This is borderline, if not full-on Armory camping, and either blocks out a RED or forces them to destroy the Dispenser and alert BLUs. The latter goes against the principle we've been following of rebelling REDs having anonymity, like how kills by REDs are hidden from the killfeed.
Another thing I'd like to point out is that placing buildings is similar to Sticky-trapping (which we don't allow) in the way that they can be operated separately from the player who places them. Imagine a Demoman Sticky-trapping a secret ammo spot vs an Engineer blocking it with a Dispenser. It's not very different.
Rebuttal: We can just disallow this type of behavior then and enforce against it. I mean, a lot of things in Jailbreak can go wrong already, so is this anything new?
Yeah, but to what gain will adding Dispensers be over these new issues that staff will need to deal with, on top of the already long list of things that could go and do go wrong in Jailbreak? So Engineer is slightly less useless? As I said in my original post, yes Dispensers could serve useful, especially on days such as War Day. However, as Kokostal pointed out, Engineer will still be pretty useless otherwise. Furthermore, even with Dispensers, Medic is a more mobile and effective alternative to healing.
The only way Engineer (without being completely revamped) could be a viable class in Jailbreak is if he could build Sentries and Teleporters, but those can't be allowed for obvious reasons. He is sort of just doomed to be useless given the nature of Jailbreak.
Needless to say, I'm now voting -1.