Just my own most recent experience... a couple of months ago I was returning from Asia via IAH to JFK. The JFK fare was cheaper than the EWR fare, as fctsty referenced earlier.

I got to IAH, and went to the gate where the Newark flight was boarding. No checked bags or anything that might complicate things. I asked very nicely if they would put me on standby for this flight... the agent seemed like she was typing me into the standby list, but then looked at my ticket and said, "Oh, you are going to JFK. You can't standby for EWR." I just accepted this and moved on.

An hour later, my JFK flight was oversold, I took the VDB and got routed into EWR instead.

Moral of the story - It seems that even if there should be ulterior motives, it may not work.