It's a subtle and clever foreshadowing, isn't it? As someone pointed out in another thread here, the credits on this site give away a crucial plot point next to Merv's name:
"Elevator killer
.

The thing that amazed me about Merv's cameo near the end of the movie was, it was actually set up earlier in the film! At one point, someone had the TV on, and there was Merv so the identity of the elevator fiend was someone who had appeared (if only briefly) earlier in the story, and not COMPLETELY out of left field.

Merv's hilarious in this film, but I wouldn't enter an elevator with him. Well, at least not while he was alive. On second thought, especially now.

Merv's cameo was absolutely brilliant and without a doubt one of the funniest moments in the whole movie.
"We're all part Shatner/And part James Dean/Part Warren Oates/And Steven McQueen"