I see that most people have Canons. On the sidelines at sports games nobody really has Sony/Minolta gear. Also the point was brought up about more used lenses, and I've heard you can rent Canon. It really comes down to the fact where I see more Canons/Nikon mentioned anywhere on forums that anything else, and it makes me wonder if there is a real reason for it.
I think we're talking about two different things here. Yes, if you shoot Sony/Minolta (or Pentax or Olympus) DSLRs these days you're going to be the "outsider" for sure. Some are ok with that and might actually prefer it, but others don't. Yeah if you're with the two big players it's certainly going to be a lot easier to find used stuff, or sell it yourself. I'm in the process of downsizing my lens lineup and have easily sold 3 lenses in the past few weeks without even really trying. I just put one in the mail today and have two more to go.
If you're interested in shooting sports, it certainly makes a lot of sense to shoot Canon since the sports market has been owned by Canon since the early-90's when their AF was superior to Nikon's. Canon has a much more complete lineup of telephoto lenses, and they're often thousands cheaper than the Nikon equivalents. In fact they're so much cheaper that some Nikon guys will just buy the Canon lens and a cheap body to go with it, and will still come out ahead vs having bought the Nikon lens. If you make friends with someone who shoots sports and they're shooting with their 400, but have a 300mm f/2.8 as a backup that they're not using, you won't exactly be able to mount that puppy up to a Sony body if you want to take it for a whirl either.
As far as lens rentals, yes, you can rent Sony/Minolta lenses. One quick Google search took me to
www.alphalensrental.com and from the looks of things they've got a pretty good selection of lenses. It looks like they're the only company that rents Alpha stuff online though, so if you need something fast and they're out of stock you're probably screwed. There's a good dozen or so places that rent Nikon and Canon gear.
If you're planning to move up to the big-time equipment, yeah it probably makes more sense to do it sooner rather than later so that what you buy today will still be compatible with whatever body you upgrade to tomorrow.
BUT... just looking briefly through the alpha lens lineup alone and then the Konica/Minolta stuff, they
DO have a pretty complete lineup of lenses there. There honestly isn't that much missing. And if you go to
www.keh.com there's plenty of used Sony/Minolta gear floating around too. In fact I just did a tally the other week for curiosity's sake and there were 46 pages total of used Nikon stuff, 32 for Canon, and 27 for Minolta/Sony. Only 10 for Pentax. That's just old film gear, not counting newer digital-only lenses. So there's plenty of stuff floating around. There was almost as much used Minolta film gear as there was Canon! Also in my local pro shop there's usually a pretty decent selection of used Minolta gear too.
I made the jump to DSLRs partly because I wanted maximum selection of lenses and wanted to be able to play around with a lot of stuff, so I stuck with the big two. It's also nice to know that if one day I decided to take my photography to the next level and start doing it for pay and want to get a full pro body that most of my stuff will still work, or that the stuff that won't I'll be able to sell easily. And that's a big thing with the Sony/Pentax/Olympus stuff. Their consumer level equipment may be just as good if not better than what Canikon makes, but there's only very limited upward mobility if you want to move up.