There was a time when I was looking for a nice 50 prime for my 7D, but I didn't buy the Canon 50/1.4 because I knew that it is too soft at 1.4 so wouldn't use it wide open anyway. I bought the Sigma 50/1.4 instead, it was very nice, pretty sharp at 1.4 even on a crop body. However, to make a fast prime really shine, you need a FF camera. Some say that Canon 50/1.4 is pretty decent wide open on a FF camera.
Reasons why I bought my 40mm:
[nice optics and compact design]
[smooth and silent focus ring]
[good price and it fits FF]
[I use a flash for low light, not a wide aperture lens]
[FF DoF seems to be much thinner than APS-C, so f/2.8 is fine]
[when I want a shallow DoF I reach for something like 85/1.8 anyway...]
[it's a nice high-tech EOS body cap , now I can take my camera everywhere (in a small case) and not look like a tourist ]
Hmmm, it's funny seeing a lot of people mentioning that 'if I want shallow DoF I go for the longer lens'
That's partly true. For a long lens to work it's magic your background has to be very far away. For confined space where camera/subject/background are not in big distance, a long lens won't achieve any more background blur than a wide angle / wide aperture lens.
And, background blur is not exactly same as shallow DoF, you can have deep DoF while having more background blur by manipulating subject/background distance, aperture and focal length. but you might already knew that.
So the point is, aperture and focal lens are different things, you can't substitute one for the other, and wide angle fast lens is wide angle fast lens and will not be replaced by anything else.
I understand the perspective thing and that's just my opinion. I like 85/1.8 better and it is not that long on FF. I never felt the need for having all of the fast primes in my collection (24, 35, 50, 85, 135, 200). What I want is 35 + 85 and 40 for the reasons I've mentioned before. However, I would get all the L primes if I could afford it ... even if I'd rarely use half of those. Before 40 came out, I was considering CZ 50/2 Macro for "do it all" lens, but my 150Macro is just too good to lose this double duty lens competition + it has AF. So, for now, I'm just trying different options, but it seems like 40 is here to stay
P.S. I really like that 40 has a hard-stop on infinity.