I've bought a few games on Windows Phone. I would much rather pay a dollar or two and get the whole game than get it for free only to realize the game either a) pops up ads in my face every 30 seconds or b) is designed from the ground up to be so addictive that it makes people pull out their wallet to purchase $20 in game items.
Not every game is going to be worth it of course, and each person is going to have their own threshold for what qualifies. I'm a small time developer myself so perhaps I'm biased but if I see a game that really peeks my interests, I'll pull the trigger for a buck or two. Even if I can only get a few hours of entertainment out of the game, that's a very good value for the cost. The market is pretty saturated with clones of existing games, even on Windows Phone, and it's nice to support someone who makes something a little more original.