I think the point people are making is that in the time it takes to earn a $15 game, playing games you would not normally play you could have more than likely made half a weeks to a weeks salary. In other words, don't use the site just to score steam games as it requires a lot of work (if you don't enjoy playing all the games, and I doubt most people do, then it is a tedious task.)
Agreed that people shouldn't use it just to score Steam games. All I was saying is their target audience is people who like to play flash games anyway, and in time maybe they will earn a Steam game. But it's not meant for people who see playing the flash games as a chore or a task to get Steam games. It's a reward, not a payment.
I personally registered on the site for the sole purpose of earning Steam games. The key perk of the site that you're excluding is that you can complete offers, as you can with any other rewards site, to earn the coins and thus the games that much faster. The folks at Tremor Games must be happy that people would contemplate this due to an assumed substantial financial compensation they receive in turn, so I do feel that rewards and payments are interchangeable in this context. The reward is payment for completing an offer that benefits the owners of the site, much like how merely visiting earns them money from site ads. We're getting paid to visit the site and earn coins that we can use as currency to claim rewards, which makes us feel accomplished. It's only in common word usage that we feel some sense of achievement, but it's still essentially payment for helping out the site owners.
Another factor is that, if you play the games only at night because you have trouble sleeping, much like I do, you are still working towards a reward. I can buy games normally, sure, but if I can get them for free through a trial or by playing several games to kill boredom when my insomnia kicks in, then I choose the latter.