I would approve, what hank said...
When I remember my "starting with addons" it was "MetaMap". Just because it provided some information/features, that I liked.
When I got started with "styling" my UI, it was Perl and then Pitbull. Today I'm trying to create aestetically mathing UIs with a mixture of oUF-layouts and "self and only for me written"-addons which are styled the same way. There is no ground-breaking thing ever.
I often tend to say: "I'm good at tanking, because my UI is optimised for this task, I see everything I need to see when I need to see." But on the other hand, there's a girl in my guild which uses only two addons: Healbot and DBM and she is performing extraordinary well.
So I conclude it is a matter of "getting used to" and "choose what you really need".
I'm proud to say that I haven't used AtlasLoot throughout WotLK, but running PUG-raids make me very sure, this is the only addon some guys use.
It will be very difficult to measure the impact of addons on the game.
As a matter of fact, Blizzard included some cool stuff into their game, which first was provided through addons, thinking of MoveAnything or perhaps the new, grid-style raidframes.
On the other side, the UFs haven't change, but in MY eyes they're just bad in terms of functionality and they are not fitting my personal sense of beauty (talking about player/target/focus).
I don't really get, what is the focus of your work, but you will have to make sure to ask more people than the addon-communities here or elsewhere, just because the people in this coms are just "mad on addons", we're using them where we could. But we're not representative for the whole community of WoW-players, who will only use a very distinguished sort/amount of addons.
I somehow got lost... Could I make any point of sense?