Can I ask why you dispise iTunes so much? I find myself prefering it over Zune, but, I am allways open to changing my mind...
Two friends of mine are huge Mac friends, they saw me using Zune one day in my home and were clearly enthralled by its colors (which iTunes doesn't really have or didn't, not sure about the newest one) and that when you played music it would randomly show album art and the artist currently being played. They loved that.
They had that same 'googly-eyed' experience watching Zune play that we all had when iTunes first debuted because lets face it, iTunes compared next to WinAMP (on a visual level even with many skins) is far superior. As clunky as iTunes can be (especially on PCs) it was vastly 'speedier' than many other options, especially when it debuted.
I prefer Zune/XBOX Music's interface to that of iTunes. Its faster, more streamlined, looks better, and on the PC run so much better than anything out there. The only advantage that iTunes has (which admittedly is a huge advantage) is there catalog of music.
But, to answer the OP question - the advantage for me in choosing WP is that I like it more than iOS or Android. The fact that WP (and Android) have physical back buttons makes me working with our family's iPod Touch a nightmare at times. One button to do everything isn't always the best option. Its one of those things that always infuriated me about using Macs after using PC mice with two buttons.
I prefer WP because it is very quick. For example, using the ESPN app on my Focus I can't ever read the little 'factoid' at the start of the screen because it doesn't take much to get into it. However, I can read it (sometimes) at least -5 or 6 times on my Galaxy Tab 10.1 when I open that respective ESPN app.
I love the look of WP to the grid of icons in iOS or the customizable grid of icons that is Android.
I think WP does a lot of things better than iOS and Android. I also love that I can delete anything on the phone and quickly. Android doesn't seem to want you to delete anything. Especially with 4.0 and 4.1. Having to go through multiple page clicks to delete an app is tiresome when in WP and iOS you just 'press and hold - select delete/uninstall/hit the X.'
For me its always the little things. WP, imo, looks and runs better. I get envious at times of my friends' vast selection of premier apps, but I can get by.