Hoi,
When you divide people up in groups, when you single out the ones "most valuable", you in effect divide the community. Whatever you base your metrics on, there will be sound arguments to deny the point of view. When it is about the number of edits, it is clear to the pure encyclopedistas that most of the policy wonks have not supported what is the "real" aim of the project.
When you label groups of people, you divide them and it is exactly the egalitarian aspect that makes the community thrive.
But this isn't about labeling people for the rest of time and saying that this is how they are defined *on Wikipedia* -- it's about saying how do you study people who regularly contribute to Wikipedia, and as a part of that how do you define the group that you are studying, which is an important question for any research study.
Given that it's impossible to study every contributor to the project in every study, and since many researchers are interested in why people who spend a lot of time or effort working on Wikipedia do so (and what exactly it is they do), this is a very relevant question for this list.
--phoebe