The advent of free-to-play has done a lot of weird things to the gaming industry. Back in the day, it was simple - you gave some money, you got some game. Now, here are the models I can pull off the top of my head: the game dev gets paid in attention and future interest in products, the game dev gets paid in ad revenue per play, the game dev gets paid for the second half or last two-third of their game, the game dev gets paid before they even start development through Kickstarter, the game dev gets paid through in-app purchases.
That last one (the one that Jetpack Joyride uses) has a lot of latitude in how it can be applied. Cosmetics, extra in-game currency, bonus levels... They all boil down to one question: Can you 'finish' the game without ever making a purchase?
Of course, this means we have to define how a game is 'finished,' which is increasingly nebulous as mobile gaming's focus on pick-up-and-play has created a resurgence in arcade-esque endless games. Still, most games have some sense of progression, usually through the purchase of upgrades or the completion of achievements. Jetpack Joyride (where everything can be purchased with the in-game currency, which collects slowly, but not so slowly as to make buying things impossible) can be completed without making a single purchase. Which is good, I guess, because it avoids the upsetting bait-and-switch thing that happens when a 'free' game starts piling on inconvenience after inconvenience - which can, of course, be alleviated with a cash purchase, just as you're getting into the meat of the game.
Here's the problem, though: The major gameplay of Jetpack Joyride (which you should totally play if you haven't, it's an amazingly well crafted little thing) is collecting coins, and the major part of the game's meta-structure (that is, the system of persistent upgrades that gives you a feeling of progress) is deciding how to spend those coins. And the only way you can give the game-makers money? Buying coins.
If you think about it for a second, you'll see the paradox. If I want to compensate the game devs (and I do, because they've given me a lot of fun), I have to cheat myself out of some gameplay. I have to make some of my upgrade choices meaningless by filling my coffers with purchased lucre. It feels like cheating, and that's why I balked from, essentially, giving Halfbrick Studios a tip for the enjoyment their game gave me.
Basically, what I'm arguing here is that the free-to-play elements of a game should be disentangled from its progression structure (this is something that goes back to the Dead Space 3 "Pay for Crafting Resources" thing from last year - if earning resources is fun, then don't give me the option to pay to take away the fun, and if earning resources ISN'T fun - why is it in your game?). Include them, certainly, but in their own separate areas - cosmetic items are ideal for this. Hell, I'm even fine with selling levels, as long as you don't go crazy with it. But when games like Jetpack Joyride merge their for-pay elements with the in-game activity of gaining money, they're going to end up disincentivizing either gameplay, or paying. And a game this fun doesn't deserve either.