In all seriousness, why does this debate even come up? But if we must:
Dream Theater are 5 guys.
Incorrect. Dream Theater is a collective noun. As such, although describing a collection of multiple things, it is itself singular, just as a
flock is a singular collective noun describing a group of sheep,
Canada is a singular collective noun describing a group of Canadians, etc. Hence, the proper sentence constructions would be:
-"The flock is by the river."
-"Canada is the nation to the immediate north of the United States."
-"Dream Theater is on tour."
They are a band. They are Dream Theater.
Yes, both of those sentences are correct. But that is because the immediate subject of both sentences is different than in your other sentence. The subject is not "Dream Theater." The subject of the first sentence is "they," which is plural. The subject of the second sentence is also "they," which is plural. Both refer to the plural separate members, not the singular collection of members. Similar constructions to my above examples would be:
-The sheep (as opposed to "the flock") are down by the river.
-Canadians (as opposed to "Canada") are our neighbors to the immediate north of the United States.
This really isn't even open for legitimate debate. So can we stop now?