Unfortunately, to get concrete data about the growing of the downloads in the Microsoft Windows Phone marketplace is a very hard job. So it is good, if you have your own concrete data from the marketplace statistic to measure the Windows Phone 8 App relevance.As an indicator, I've even used my Windows Phone 8 app NFC Tag Manager. The NFC Tag Manager app has reached a download number of around 1200 in the first month. A number, which I never would have expected. For me so a clear indication that the Windows Phone 8 market is growing.
So build now your own Windows Phone 8 app.