In my opinion a great reason for investing time in wp7 development is first and foremost the fun developing with your favorite language and the best development tools on the market. Silverlight and Xna coupled with Visual Studio 2010 blows all other mobile
development platforms away and it does so single handedly. Out of great fun developing will come great applications for the windows phone 7.
Everything else is just a guess. When gartner estimates the wp7 will become the 6th most used mobile platform then this is nothing more than a guess. They have no idea how windows phone 7 will be adapted because nobody does at this point of time.
You can keep in mind however that investing time into developing apps for windows phone 7 means investing time in .net, c# and silverlight and/or xna. And this is an investment very much worthwile because it means you can target your application for windows
and for the xbox as well. And this is a very big market already.