You don't have the right to do that though; the fact that you might enjoy a game that has been slated is irrelevant. It should be a case of "no demo - no purchase". If you are buying games that have no demos then the devs/publishers will never learn.
If it is unacceptable, then people shouldn't be buying the game in the first place - that is implicit acceptance!
I don't necessarily disagree as to whether GTA4 deserves your money or not; in another thread you did describe it as a "great game" though. I'm not incredibly rich but I don't play every game on the market either, the chance would be a fine thing

Realistically a PC gaming habit should only cost about £50/month on average except for people with a lot of free time or those that insist on buying all the big titles the moment they come out.
Finally I've probably played games I shouldn't have in the past (so I'm not one of those holier-than-thou saints) but that doesn't mean to say that I consider it to be justifiable.
I never claimed piracy was legally correct, but it is morally, so long as it isn't abused. I have, like many; downloaded a game to see how it was, and then uninstalled it if it didn't run well or if I didn't enjoy it after the first hour, so they are in actual fact just demo's. The world isn't as black & white as you claim sir.
A brilliant example would be Skyrim, I owned that on the Xbox 360 and loved it, and now I could afford a new PC and bought one, I sold the 360 on with its games to help with the hole it left in my funds.
Am I wrong for pirating it on PC to see how it runs? Am I wrong for uninstalling it when it ran like ****? Am I wrong for installing it again after 1.5 to check out the memory and coding optimization? It runs almost perfectly on my machine since 1.5, so I'm glad I gave it a second chance.
I never played past Whiterun, I did a lot of modding and all in all had about 2 hours on my save, half of which was mod testing. After having a chance to see what the performance was like, and having the chance to test out the 1.5.4 and 1.6.9 patches for their coding optimization, I'm going to purchase it when I have the available funds.
As for GTA IV, why would it deserve my money when it runs so pathetically despite doing
nothing hardware-intensive except generally being a badly coded port. There is no excuse for it, none whatsoever.
I knew before pirating that those two games were amazing ones, yet neither of them have a demo available. Do I not have the right to see whether my machine can actually run them before I buy them? Should I miss out altogether just because it is 'wrong' for me to use an illegal torrent for a demo? Or should I play Russian Roulette and hope to god the game doesn't shoot me in the face and have me waste the money?
If this was the first time I'd seen you on these forums, I'd probably dislike you for your views, but alas I have no quarrels with you good sir, I just wish you'd be a little more open with your views, it is in no way shape or form fair on the consumer to not have a chance to demo a game, especially on a platform like PC where performance can be an issue. I will be pirating GTA 5 as soon as it is available, and I can guarantee you I won't be playing it past the second mission before buying it, nor will I buy it or play it for long if its yet another mess of a port, but Max Payne 3 has given me hope (a game I blindly paid for and thankfully have not regretted, as it runs great).
I have some morals, I won't go and play through a game I have pirated, I only do it to see if I can run it and if I enjoy it, which I have every right to. The same as you have the right to test drive a car before you buy it, or view a house, or try on a piece of clothing in-store. I have never played any game I've pirated for any more than 2 or so hours, except The Elder Scrolls Arena & Daggerfall, but that was only because DOSBox didn't work for me (they are both free from Bethesda's website anyway).
If developers weren't so lazy and gave us demo's, piracy figures would absolutely shatter.