Betas, PTRs and Demos oh my! Space Marine, Battlefield 3, SW:TOR, Diablo 3, WoW 4.3 PTR - you've got your pick (yes, I know Space Marine is live - and it's AWESOME) of games to select from. I'm sure there's a bunch of others available as well, these are just the ones that have caught my attention.
Now, there are certainly differences between a demo, beta, and PTR - but they're all basically designed to do one thing. Whet your appetite for the real deal. Some do it by giving you a (hopefully) polished taste, and others do it by letting you help to test out the upcoming content. Either way, it's a great way to get a feel for the new stuff by playing before the full package is released.
Too much too soon
One of my biggest issues with these is that you start burning through the actual content early. One of the biggest complaints I hear about games is that you run out of the "new" too soon. Well by participating in the free play, you're just burning through it that much faster. You also spend your time playing in an environment that may be less than stable. While instability isn't limited to non-live environments, it's certainly more prone to bugs and issues.
Now if you're actually interested in testing software and actively reporting bugs, then by all means please continue. I appreciate your efforts to make my experience better. Personally though, I do enough testing and trials at work, and I'm compensated quite well for it. I don't feel that getting access to content early is enough to justify the work that I would want to put into being a proper tester. There's also something that is kind of disappointing when you know that everything you just did, you'll have to do again when the game goes live. I get enough of that just leveling alts in WoW, thank you very much.
Demos save you money?
I know some people play demos to help make a decision about the games they want to buy. While I can appreciate this, I'm generally either sold on a game long before it's released, or I show up to the party 6 months late. I was mentally commited to buying every game that I have on my list (BF3, TOR, D3, SC2 Swarm) long before a beta was even announced. I love Star Wars, Space Marines, and the previous Diablo and Starcraft games. Until a company lands a giant turd on my desk, I'll usually stick to a few specific producers. Of course I didn't really like ME2 or DA, but who knows - I'm really hoping that Bioware delivers with TOR.
In the end
At the end of the day, these just aren't for me. While I'm not severely limited in the playtime that I have, I'd much prefer to actually progress my character than spend time in an environment that will soon reset, and I'll have to start again. I have no illusions about ever being a Masters SC2 player, getting a world first WoW kill, or topping the kill charts in BF:BC2 - I'm just looking for a few hours of entertainment.