I will admit that with the 5D III the dominance of primes when it comes to low light has been compromised by the higher usable ISO range.
I'll still use my 85/135 lenses for subject isolation in portraits, but the 35L was my go-to lens for indoor ambient light shooting and nighttime walkaround, and I wonder how much I'll use it in those situations now that I have a 24-70/2.8 II.
The obvious question is...how often do you shoot faster than f/2.8 not because that gives you the depth of field you desire for the shot but because it's the only way to get the shutter speed you want, and you really wish you could shoot at f/2.8 or slower to get more depth of field?
With the 35L, the answer is 'often' for indoor ambient and 'sometimes' for nighttime walkaround. Regardless, I'll hang onto the 35L for a while. It's not a fair time to evaluate indoor ambient usage with DST begun and days getting longer. I'll see what happens next winter...
I recently shot a boys basketball game in response to a last minute request from a friend, who was the young boys' coach. I'd never been to the court before, so I brought a few lenses. Well, the gym was not much larger than the court, and I was shooting near the baseline near one of the corners. I had hoped to use the 135L but it was too long and I had to use the 24-70 instead. A large percentage of the shots were wide open at ISO 3200 for shutter speeds near 1/500. The zoom saved me that day because most of the pics were from 35-60mm from under the basket. The pics were good and I was impressed how well the 5D3 + 24-70 II's AF worked.
The next day, I shot a birthday party at a beginner's gymnastics gym in even dimmer conditions. The gym had a large bank of windows on one side, which made a lot of the pictures strongly side or back lit. I started with a 85mm prime close to wide open but it couldn't track the kids erratic movement, so out came the 24-70, which delivered so well for the basketball game. Most of the pics with the 24-70 were at ISO 5000 near 1/500. What surprised me was that the basketball pics were a lot better than those from the birthday party. I know it's not any one thing, but the higher ISO (and noticeably worse DR/increased noise), worse lighting, and smaller, more erratically moving kids, all had their parts for why the images were worse. The prime missed a lot, but it was better at taming the harsh side/back light due to the natural vignetting and dimmer/more defocused background due to the larger aperture (plus it allowed me to drop down to ISO 1000). In the end, I chose the zoom over the prime in both cases (IQ drops with higher ISOs but missing the shots because they're blurry is even worse), but I sometimes wish I had a fast-focusing 35 or 50 prime instead.
For more stationary subjects indoors and for use at night, I'll opt for the primes almost all the time. For shooting kids sports where I have access to the sideline (not focal length limited), zooms are great. Did I mention how impressed I was with the 24-70 II's focusing abilities??