All I have to say on this is this:
THeres nothing stopping you from playing the game you have a (generated) code for on its own, why do you want to cause trouble on steam while doing so? Thats not helping anything and you just have a duplicate copy in steam of the same game.
I was going to bring this up but... I dunno, telling people how to effectively pirate a game on the developers' forums just strikes me as bad news, even if the guys are cool with it.
Here's a simple Piracy legitimacy outline.
1. Does the game have a demo?
Yes: Download it and play it first.
No: Move on.
2. Was the demo representative of the game? Did it exist?
Yes: Buy the game if you like it, move on with your life if you don't.
No: Go on.
3. Is the game inexpensive (20$ or less)?
Yes: Move on, but consider just buying it anyway.
No: Move on.
4. Download the game or use an illegitimate key. Play the game for a limited time (we're talking maybe 10% of the levels tops, or limit to 2 hours or so.)
5. Did you enjoy the game?
Yes: Delete your pirated copy and buy the game.
No: Delete your pirated copy and move on with your life.
Maybe?: Delete your copy and have an internal debate.
That's the chart that I follow basically. Inexpensive games can wait till I have spare money. Such a low price is just a couple of okay meals, and I think anybody can replace a couple okay meals with ramen if the game looks cool enough to take that chance. It's when you're treading the line of a week of ramen where you draw the line. Expensive games need a couple of levels of demo. If there is not a demo, that is on the developer or the publisher, and they aren't getting my money until I demo the game and determine whether I like it. TB says that piracy is a service issue, and personally, I'm inclined to agree just because developers who make me happy get a whole lot more wiggle room. I'd totally consider buying Tidalis for instance, if I knew I'd get at least some play out of it, right? It's a well designed game. Thing is... it's not my thing, so I just haven't yet. That's just what comes naturally to me.
Edit: Oh yeah. If by some miracle you beat the entire game in two hours, it might be worth considering buying it given that you actually already beat it on accident. Those things happen.