Walk through any shopping mall food court and you’re bound to be offered a free sample from at least one of the restaurants. The taster is such a staple of mall marketing for one simple reason: it just plain works. This tactic has been borrowed with success by software-as-a-service (and even downloadable software) companies and as such is also common with API Products.
It's hard to get excited about developing applications if there's no payoff for your efforts. That's the general consensus arising among makers of Windows Phone applications, who are dissatisfied with the poor sell through of advertisements. With no money coming in from ads, some developers say they'll have to start charging for their apps - or quit supporting Windows Phone.