it depends on the situation too and how many people left in the pot. Sometimes its stupid but if there are many people left in the pot on the river, it can be advantageous to trap people in with just the bare nut low thinking you are getting half the pot and everyone else is calling with lesser hands.
For example, a lot of times if 4 people are on the river and one has the nut low and one has the nut high, if they can trap people in the hand who are calling with two pair/2nd nut low and such, it can get you a few more bucks. however, some people just bet and raise with the nut anything. Also, its more useful if you have the nut flush on a non paired board rather than the nut straight as you could be getting quartered and such.
I would have to know all the details of the hand to have a better idea if its collusion but there are a bunch of idiots out there and they only know they have the nuts for half the pot. they don't think about being quartered and such.