Yes they are expensive and just as costly to repair. Some docks you cannot unload at because they can not lock your truck into the dock.
Some people on this site swear by them, I am not an advocate of them. If the customer cannot supply me with a dock than I really do not want it on my truck, fast load, fast unload get ready for the next load offer ASAP.
Others will argue that they get more freight because of it but I doubt it. I have nothing to prove them or myself right or wrong. I do no this they are expensive, and the extra revenue that may or may not be there is going to be hard justify the extra expense.
Example look at the difference between what a D-E truck brings in, now calculate the difference between what the two generate in a year, divide that by the amount of what each unit costs now you get an idea of what Extra Services on a truck should net you, a liftgate will not in my opion make up that difference.
Ask people out there when you see them at the docks or truck stops, I have talked to some people who agree with me and others who love them. Those that do not like them though out number those that do primarly because of the E-Xtra work they are required to do with the low pay (we are expected to do everything for oh lets say nothing or close to it).