Gotcha. I don't see a problem with testing out a game before purchasing it, or put it back on the shelf and look for something else if it's not interesting/you don't like it. That's common sense.
But if there's Trailers, Gameplay Footage, Screenshots, Reviews, Forums, User Feedback, you can get a pretty good idea if the game is something you want, or if it's something you could do without. Today there's usually tons of coverage on each product. For free! You used to have to get less of that coverage from Game Magazines and also pay for it.
So, basically, you have the option to look at the videos, look at the gameplay, screenshots, even a first impression reviewer or a let's player, read reviews, go to forums and get a general idea "What do the fans think of it, those that really like it?" or other forums "What do other forums say about this game?". You might even make an inquiry, a new thread "Is this game any good?". If it is something that interests you, you can secure the purchase by gathering enough information. Granted that it is something you are drawn to and are interested in purchasing.
In other words, you'll be able to figure out with about 90% certainity whether you will like a game or not before you even play it, based on your previous gaming preferences or even your gaming mood (Horror game, FPS, RPG, Driving, Strategy, etc. what are you looking for? What do you want to play?).
Steam allows for refunds by the way. It's a terrible refund system that needs a lot of work, but it's in the early stages and like most things it will probably evolve. In other words, "I'm pirating to try the game out" excuse is only becoming thinner and thinner
Intinium, as now you can purchase a game*, play for an hour and when you don't like it, refund it.
Also, before I forget... the "Whoops! I finished this shitty game I pirated... was only going to try it out!" has never been a good excuse. Even less good of an excuse if you are employed with salary.
* This is what I'm personally iffy about in their Refund plans, you should get "Game timer" instead of having to pay for it (If you play for more than 2 hours, Steam seems to mean that "You like it enough that you'd buy it", meaning that you might play a game for 2 hours and 10 minutes, by mistake or accident, then you'd be forced to pay for it). Or maybe it is "Refund within 2 days"? I don't quite recall, I haven't used it.
The "Game Time" could instead function as a "Lock". You could get between 1 hour and 30 minutes up to 2 hours to play the game. When the Game Time is over, the game will Quit, and you get an Question "Did you like the game?", with "Yes" and "No" options.
If "Yes" you'd be taken to the Cart, and could proceed to checkout. If "No", you'd return to the Main Store Page. Simple enough, and "Pirate to test" would fly no more.