I'm really not interested in there being another type of employee to keep track of. I find the existing employee system to be too micromanagement-heavy and buggy. The idea of managing greeters makes me worried it would be tedious.
When it comes to realism, I normally see greeters on rides that have an upcharge queue. Free entry parks (see Jolly Roger at Ocean City, MD) usually have the operators collect tickets or swipe cards as guests board a ride. Paid entry parks (see Hersheypark in Hershey, PA) normally have someone greeting the "fast track" line to make sure guests that use it paid to degrade everyone else's experience. I don't usually see greeters for other rides. High-end theme parks (see Universal and Disney) have greeters everywhere due to higher expectations, a lower ride count, and using greeters as information helpers and front-line security people.
Also, do not forget the unsung hero of the greeting world -- the park trash bin! Nothing quite says "ride is closed" like a trash bin sitting in the middle of where people usually enter the queue for a ride. Trash bin off to the side? Great, ride is open!