I don't really focus on specific types of photography, I shoot whatever I find interesting or makes me happy. As for using 15-35mm a lot when shooting with a 15-85, it's because it only goes to 15mm on the wide end. There are many times I find myself wishing I had a lens that was wider than 24mm, but I'm not exactly sure how often I'll go that wide because I just make due with what I have. I think the 15-85 is a fine lens, and infinitely useful as a travel lens aside from its slower speed. One thing that irked me was the silver ring... which I recently found to be a sticker and had begun peeling off on my lens. I also used to use 70-200 as a walk around lens on my 7D. I love the shots I get and I find it more satisfying to use than the 15-85, albeit I've changed my walk around lens to the voigtlander 40mm f2 because its just so compact and still quite versatile on the 7D despite it being a short tele when used with an APSC. However its not the best lens to have around in many cases, especially in low light. Even with focus confirmation I often have to use live view to make sure my shots are in focus when shooting wide open. Therefore I use it when I can take my time to manually focus and get clear shots.
When the light is bad, I tend to shoot wide open with the 15-85, but in low light situations 15-85 just doesn't cut it as the increased ISO on the 7D really causes the picture quality to drop(I can still accept 1600ISO... albeit a bit grudgingly, and above that I would start balking). I've contemplated on just getting new glass, but the 17-55 f2.8 doesn't really appeal to me that much, and the L series lenses works better with FF in most cases. I probably could get a prime or two, but I'd prefer more versatility with a zoom lens. Thus the reason I'm considering the 5DIII with either the 16-35L or Tamron 24-70. I didn't consider the 24-105 as I'd rather have a faster lens for times when I do need the larger aperture settings, instead of lamenting like I do with the 15-85 at f5.6 when zoomed to 85mm lol.
I have telephoto covered with the 70-200 f4L IS and is thus the reason I thought of forgoing a general purpose zoom for the 16-35mm.
Reinzphotoz I'm basically in a similar predicament as you were, but the wider-than-24mm focal length of the 16-35 appeals to me as much as the versatility of the 24-70, and thus my dilemma with choosing a lens. Let me know what you think of the Tamron 24-70 VC once you get it ;-)
I have enough dough to buy the 5DIII, so I probably won't think about getting the 6D since the AF system of the 5DIII interests me the most. I'll most likely also sell the 7D to help fund the purchase, since I really don't have a need for two cameras and would probably see the 7D grounded. I think it'd be better to let someone else use and enjoy the 7D as I did for the past 2 or so years. I'm still considering as of now, and am holding off to see whether the 5DIII price will drop a bit more where I live (currently at 2730 USD if converted with today's currency rates)
P.S. it's not that I don't want to spend the money, it's more like trying to find the best possible solution to my dilemma and not really wanting to part with my 7D that much
As for the 6D... I like the 5DIII better(much better)~ enough said
P.P.S. I'm fully aware that the 16-35mm on FF isn't equal to the 15-35mm on APSC, it's just that I was considering trading off the 35~50mm range for 16~24mm ultra-wide angle.