Well, that's kind of the point. If you can download something that's 3GB in two minutes, it fundamentally changes how you deal with media.
There's also the issue of security that I didn't mention. Public torrents are visible to everyone, so your download or upload is an action viewable to everyone. With private trackers, your actions are only viewable to the members.
Not really in my experience. I'm a big usenet guy, but I still horde a local collection - if you're using a private tracker you don't want to be re-downloading things all the time because that'd cost you ratio and you still need to hold them long enough to upload them a fair bit. Usenet is really the closest you get to not needing to hoard, but there's still a risk with that, so you often keep it around.
I wouldn't say having something in 2 minutes vs an hour, but still needing to keep it around "fundamentally changes how you deal with media". Especially when you can freely stream a torrent using peerflix off a public tracker, but can't off a private one due to ratio concerns.
Sure, you won't max gigabit like you will on some private stuff, but unless that's an absolute need, issues with public trackers are fairly rare. In fact I've occasionally found files with only a single peer or two on a private tracker which would barely download which were much more readily available on a public tracker.