Than even the hypotetical RF 35mm f/1,2 might be not necessary?
Well, when I say 1600+ is acceptable, of course 200 would be better :-D
I used to shoot my Leica at for instance dinners out in candlelight at 1600, 35/1.4, 1/15 or 1/30. And that's not the exposure a light meter might advise, a meter might have wanted 3200 or 6400 but I just drew the line at 1600 grain and prefered to be underexposed rather than any more grain.
Starting I think with the next R they'll have IBIS with possibly 4 stops, which is 100 35/1.4 1 second ?!!? Sorry, people move around too much! Maybe 1/10 sec would be OK though.
So let's say, 800 35/1.4, 1/8 sec? People move much slower at night so could work, but f/1.2 would give 800 35/1.2, 1/10.
That sounds pretty good. And ISO is still relatively low given modern tech, so if you find DOF or image quality is too weak wide-open, or the shutter is getting too much subject motion, you can use a little faster shutter or smaller aperture and still have a relatively low-noise 1000.
Ideally Canon would come out with a super-compact 35/1.4 like the Leica 35/1.4 before the ASPH, then a cost-no-object, size-no-object 35/1.2 that's probably huge.
At night I either shot the Leica inside--where the wider angle was absolutely critical--or shot Canons on tripods, in which case I'd use Velvia (50 speed film) and 10 second exposures or whatever.
Arguably you it'd be great to also have a 24 or 28 for shooting a photo at a memorable dinner.
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Anyway I'm just rambling. In direct answer to your question, you definitely don't "need" f/1.2. But it may buy you a bit lower ISO or a bit faster shutter, which might be handy. And a lot of times you want a bit less DOF say to de-resolve other tables' diners, or the drink station behind your date, or what have you. (35/1.4 has DOF of 50/2, so 35/1.2 would be about like 50/1.8.) In short you CAN shoot single subjects all the time wide open at 35/1.2. But again with 35mm you're usually telling a bit of a story with multiple subjects, and if they're not on a plane than suddenly you'll need 2.8 or something, Depending on how common that is the 1.2 might not be used often.
I'd probably buy both a super-resolution monster 35/1.2, and a average-resolution tine 35/1.4... then carry the big one any time I was going out to dinner anyway, so I'd have the 1.2 if I needed it. I'm a geek. OTOH I'd have the tiny 1.4 mounted on the camera in my backpack and end up taking most of my shots with that I think, just because I had it with me.