The silly thing is that you only use wide open -
A. to isolate your subject with selective focus, in which case your subject is the only thing in focus at all. Or rather, your subject is more in focus than anything else. With the shallow depth of field of a f/1.5 lens, we are talking about eyes only at close range. With a 50mm lens, the eyes are a very small part of the frame, so any judgement about sharpness wide open is based on a very small region that may or may not be absolutely and entirely precisely in the actual plane of focus.
B. because any smaller aperture would require a shutter speed that would certainly cause blur from camera shake. At which point "sharpness wide open" is a non-issue. The same lens at a smaller aperture would be sharper, but you could not use a smaller aperture without a slower shutter, inducing camera shake and/or subject movement. So how sharp the lens is at that point is a lesser factor. A slower lens would certainly be blurred, and sharpness is already compromised by the slow shutter speed. We cannot know that said shot would be sharper with any other lens.
So we have two cases of actual wide open use. In one, we actually desire softness across much of the frame. In the other, we are pleased that *anything* could be sharp at all. I would rather use the lens that delivers a nice look to the images than one that is theoretically sharper wide open.
One aspect about fast lenses that many people forget is the issue of vignetting. If you see vignetting at 2.8 on your regular 50mm, a faster 50mm will produce less or no vignetting at that same aperture. If you shoot a fair number of shots at f/2.8 and regularly see vignetting, a J-3 is a good solution regardless of whether you intend to use it at f/1.5. We all use our J-3's wide open just because we can, but it isn't for sharpness. It's for the narrow DOF and the speed. It's like taking portraits with an 85/2. We don't do it for maximum sharpness, we do it for the effect. You don't need an 85/2 for shooting at f/8, but you can't use an 85/4 to take portraits at f/2.
I drive a RWD car with above average horsepower. I don't do it so I can flat-foot the gas at every light. I do it because I like driving it. The things it lets me do with the car at half-thottle on a whim trump what most people consider when buying a car. I win, IMHO they lose. I don't have to use it to full potential every day to rationalize the cost. And I don't have to justify it the same way the guy with the four-banger Camry does. I consider driving an art. If you consider driving a metric of dollars per mile, you will never understand where I am coming from.
I feel the same way when talking to people who always ask about sharpness wide open - you don't get it, yet 🙂