Because you are starting to tell the pilot how to fly. There are ramifications for what you are doing (maybe not in your 1500fpm scenario) which I don't think we can be expected to keep abreast of. eg. how much speed will drop off for a given rate increase, what mode change is required to control the climb rate and what effect will that have, how economical is that solution etc. etc.? We are supposed to service our customers as well as possible these days, and I'm not sure we are if we are taking control of how the a/c fly to such an extent.
I'm sorry, I don't understand your post at all Ferris. You say it is about customer service, yet pilots here have said that when told to "expedite" many pilots perform what is tantamount to an emergency climb or descent. You say we can't be expected to keep abreast of how a pilot flies their aircraft, which is exactly why I suggest a climb rate, and leave it up to the pilot to advise if they can't comply. I consider that to be positive controlling, rather than plucking a misunderstood word out and hoping that will solve the situation.
As you did say, the term needs to be clearly defined, the world over, and until that time arrives, I won't be using it. Considering every man and his dog is using it as a routine instruction, I will also have to think up a new phrase for (heaven forbid) my own emergency "I screwed up" situation. I guess rather than expedite, I will have to use the old chestnut "climb like your life depends on it..."
I know it means very little in the big scheme of things but a quick straw poll at the centre showed that about 50% of guys were vehemently against using expedite, other than in an emergency situation, 30% thought it was fine to use, and the rest couldn't give a toss either way. I know, statistically an irrellevant number of people to ask, but it just highlighted how split ATC'ers are over this issue, at least in this part of the world. I would be very interested to conduct a poll of pilots, asking them what they understand the term to mean and ask them how they fly the aircraft after being told to expedite.