I've found that collectors and curators of photography generally know more about photography (as a subject area) than *most* photographers. It's like in baseball, which has a huge number of dedicated fans, and a very small coterie of people who can actually play in the big leagues. What they know about different - the fans know about statistics, abilities, etc., the players know how to track down a fly, how to pick up a fastball, etc. But what about high school ball players? They don't know what the fan knows, but they also don't know what the big leaguer knows. And that's where most photographers -- like 99% -- are. The don't make great art, but neither are they (usually) as knowledgable as a dedicated collector. To say that a wedding photographer, knowledgable as he may be in his craft, knows more about photography in the wider sense (aesthetics, history, personalities, print quality, etc.) than a dedicated collector, would usually be wrong. There's no reason a wedding guy *should* know all that stuff. He's a craftsmen, not an academic. It's like suggesting that a guy who makes bookcases should know more about furniture than the curator of the furnishings period collections at the Met, or an antique dealer, or whatever. They're different fields of knowledge, and the academic one is broader. Further, it has been my experience that most academics, curators and collectors are also photographers, and sometimes quite good ones. Some photographers, of course, also have advanced degrees in art history...it's not all just one thing.
The assumption that most people collect for investment reasons is also wrong. There are much better investments than photography -- in fact, almost anything (rational) is better. Most people collect because they love the art, and they know a lot about it. If they're going to pay a lot for it, they usually want to know why - and that's where art advisers come into it. They can look at things like limits on the editions, precise quality concerns (is it archivally fixed?) and so on. There's usually not a concern about making a lot of money, but just seeing that the piece holds its value, which is a normal thing, if you're going to pay a lot more than its obviously nominal worth. I saw an Ansel Adams "printed later" (1970s) "Moonrise" at LA Art this year, with an asking price of $150,000. Why would anybody pay that? That's what a collector wants to know; it doesn't have anything to do with greed, it's simply a reasonable question. If I needed a garage, and somebody offered to sell me one for $150,000, I'd ask the same thing. Why that price, and not some other? Is the garage in good shape, or is it going to fall down next year?