Just look around for a 24-105 that comes with a kit on 5Ds. 15-85 sort of have many negative specially on zoom creep. In the long run, that zoom creep might get worse. Plus it's not weather sealed. Distortion is another factor worth considering on a 15-85 add up the lack of hood which canon sell for a premium. Having a walk around lens in my perception is having more reach as possible more than wide view since if you really want landscape get a tripod and go for your 18-55 kit lens. Constant F4 and the macho red ring - lets face it it's that cool.
True the 15-85 has distortion at 15mm, but by 24mm it's resolved and is less than the distortion on the 24-105 at 24mm. And the distortion on the 15-85 is easily fixed in Lightroom 4 and other software. The zoom creep is not that bad and certainly no worse than most other zooms with heavy front elements. There have even been reports of the 24-105 creeping. As to the creep getting worse, the 15-85 has been around for quite a while and I have done extensive searches on this lens before and after I purchased it. I have yet to find a single report of the creep getting significantly worse over time. Usually the creep develops in the first few days or weeks and just stays that way. True, its not weather sealed, but this is not a thread about which is a better lens or a better built lens, it about which would better serve the needs of the OP.
With regard to your comments about using a tripod and an 18-55 for landscapes, I'm completely befuddled. First, when I'm out hiking to take landscapes I generally don't carry a tripod or extra lenses. so I'd have to live with the 18-55. Second, I wouldn't be able to use a polarizing filter on my 18-55, Third, the IQ of the 18-55 is not as good at the 15-85, Fourth, 15mm is way wider than 18mm for landscape use, and 85mm is way longer then 55m. Fifth, the 18-55 does not have FTMF, which I often use.
Your final comment "Constant F4 and the macho red ring - lets face it it's that cool." is a two parter. First, f4 is not that fast, and for outdoor landscapes in the light I shoot it's irrelevant. The second part of your last statement is also completely irrelevant. I'm not trying to impress anyone. Using the term "macho" in regard to a red ring? Yeah, its a real chick magnet.