I am going to chime in real quick and say that I think signs would be a good idea for ATV/SxS trails. Here is why. I our system actually uses gates on the ATV/SxS trails. That is to keep the trucks with mud running setups out. The way it is done is that the trail is blocked by the gate and there is an area just wide enough for a SxS to get through on the one side. So riders actually have to go around a closed and locked gate to legally ride that section of trail.
I do not remember the signage at the gates, but I know this system is used in other areas too. So it does add a bit of confusion. Some may argue that is a crappy way to keep trucks out and but let ATV/SxS's get by. I am not going to get into an argument with that. However, it actually serves it's purpose very well and is way less expensive than trying to put something else in to accomplish the same task.
Plus, a rider unfamiliar with the area comes up to a gate, how do they even know they are on the right (legal) side of it if there are no signs? If there are tracks going around the gate, that only makes matters worse.
I know that having, reading and understanding maps also goes a long way towards keeping folks out from where they are not suppose to go and that signs will not keep everyone out, but seems like a pretty simple thing to do to help keep some out that are just confused.
-John