Difficult question, from a dev perspective the WP7 development tools are very easy to use and you can build excellent GUI's using Expression Blend. AIR would mean moving away from the established Metro UI themes for the sake of cross-platform compatibility.
Any reason to encourage new developers into the marketplace is a good one, and for that AIR support would be great, but as an existing dev the prospect is far less appealing.
I agree with you Kall. The buzz on mobile is all about cross-platform portability, so I think this will be an important move for WP7. In fact, I believe we are demonstrating AIR on WP7 in today's (April 21st at 3pm central) WebORB webinar. Please join us if you can. Here is a link to register: https://www2.gotomeeting.com/register/810889819
Every time I install Adobe Air on my computer by accident, I immediately uninstall it. I've never used an Adobe Air application that did not seem ugly, clunky, kludgy, and jarring. This is all just personal opinion of course, but I prefer the consistant Metro look across WP7 apps. I don't want apps that all look different (and ugly).
I can see why it would appeal to developers though.
I hate Adobe AIR with a passion! Agreed with Waycoolkennel with how it barely works on Windows and if it does you will lose a good chunk of RAM. The only app I'm prepared to run on AIR is The Sun model (forgotten what her name is) where she pops on screen and then walks off :P
Im not the best qualified to answer this question but in my opinion it would be FANTASTIC !!
I think Adobe have done an EXCELLENT job with AIR publishing to Android (I must admit to not yet having tried publish to i Phone) and if in AIR we could choose to publish to WP 7 (if this is even possible) then AIR would be one system ernabling you to publish to almost every OS in mobile.
Air is decent problem is hardly anyone is using it for something other than some useless and redundant twitter or Facebook client. Also the air development tool (IDE etc) are rather expensive. I've moved on from using disparate ill integrate tools for development.