I'm thinking that either I am misunderstanding you, or you misunderstood me. In that particular post (mine, that you quoted) I don't believe I mentioned zooms at all.
Correct. My mention of zooms was in comparison: this is how much you would spend on this lens, and these are other lenses that you can buy for similar money.
I did say that the 50mm prime was roughly the "film equivalent" of a 75mm lens, in answer to a question; and I said that I didn't think the argument for the 50mm in particular is all that compelling (which I realize I'm in the minority on). Personally, I ponied up the extra $200 for the 35mm f/2 instead of the 50mm f/1.8 - and I think I made the right call.
Which is good to hear. I'm not saying that the 35mm f/2, or indeed any other cheap-ish prime, is not worth the money. I'm simply saying that, for the price you pay, you cannot get a better quality lens than the 50mm f/1.8.
Is it useful for everybody? Probably not. But it's so cheap, handing over the cash for it to find out is no big deal - unlike some primes which cost tens of thousands of dollars (super telephoto, so highly specialised, but anyway.) Even five hundred dollars for a lens is getting into territory that a lot of people are reluctant to explore.
BTW does Canon do that (have a line of lenses that only really work with the APS-sized sensors like on the Digital Rebel)?
Yes. Any Canon lens with the label "EF-S" is specifically designed to work only on bodies with a 1.6 crop factor (with the exception of the EOS 10D, which, although it has a 1.6 crop factor, does not have the modifications to the EF mount to allow it to use EF-S glass.) There are currently just five lenses with this specification: the EF-S 10-22mm, the EF-S 17-55mm f/2.8, the EF-S 18-55mm, the EF-S 17-85mm, and the EF-S 60mm macro.
But in any case my original intention was simply to say that I don't personally think the 50mm on a dSLR is quite the be-all and end-all that some people think it is. I freely admit that people can and do disagree with me
and that what's best for one person isn't always best for another.
I don't think anybody is saying that it's the be-all and end-all of lenses. All I'm saying (and I think a fair number will agree with me in this) is that the 50mm f/1.8 is so cheap, any given photographer can afford to plonk down the cash and try it. It's not useful for every shot, even on a film (or film frame) SLR. The reason it's recommended is because it is a superb lens at a very cheap price point; even if you only use it a couple of times a year, it's worth the money, simply because it's so cheap.
Before I bought that first prime, I did an experiment. I walked around with my D70 and the kit 18-70mm lens, set at 50mm; and looked for the types of shots I thought I'd likely be taking using a prime. For the majority of them, the 50mm would have been fine; but there were a non-trivial number where it was bringing me in too close - and it wasn't always a simple matter to move further away in a timely manner. With the 35mm I don't have that issue, and I can move in faster than I can move away.
It's all about getting a lens that suits your needs. A 35mm prime fits your needs better than a 50mm prime, and that's great - you know your shooting style, it works for you, no problem. I just find it much easier to recommend a $AU145 lens than a $AU400 lens when I don't know exactly where the budding photographer will go with his or her photography down the road, and at that price, I strongly believe that no semi-serious photographer should be without it.
