i guess i should've said, middle focal length f2.8 prime...obviously once you go long it's good...but a 40mm f2.8 prime on a crop to me doesn't make sense...you can buy a zoom that will cover that
You can buy a zoom that covers any singular crops focal length....
You are missing (or ignoring) his point - zoom with f/2.8.
To add to what Sirrith pointed out.
(With the sharpness of many current zooms) Unless there is some other reason (macro, TS, IS), a single focal length with f/2.8 seems silly to some (especially if they already have a f/2.8 zoom). Yes, there is a size difference, but many a f/2.0 and f/1.4 primes are are also smaller than a zoom (but they are at least a stop faster).
So for me (I do own an f/2.8 zoom), in the case of the 50 vs 40. I see no reason to get a 40mm f/2.8 versus my 50mm f/1.8 MKI. I would rather have a noisy but over a stop faster f/1.8 than than a quieter f/2.8. Yes, the 40 is smaller, but to me not that much smaller (when we are talking 2-ish versus 1-ish inches). An f/1.4 would be even better but Canon's is prone to AF failure, and a Sigma is larger. An f/1.2 would be even better, but it is also large (plus I can't justify the price for that lens).
Yes, I can see some (just not me) wanting even the one inch difference. If I shot more video, I would likely go 40mm f/2.8 over the 50mm f/1.8 because of lack of noise and such. I'll even say that if I saw one for an excellent price, I would probably get it. But chances are it would get used less than the 50mm.