Yup, that's a perfectly valid question and developing using Xamarin studio on the mac probably is the answer for many people.
For me in particular though, I have years and years of 'muscle memory' with Visual Studio and even though I have used Xamarin's UI (MonoDevelop for previous Monotouch versions and now Xamarin Studio) for over a year and am reasonably proficient with it, I'm still much faster inside Visual Studio/Windows environment. Just silly things like hitting F5 to debug instead of remembering to use Command + Enter. Ctrl + C for copy, instead of Command + C etc. It just means you don't need to switch gears as much each time you switch platforms.
Plus, there's some things that Visual Studio just does better than the Mono UI (I'm not putting Mono down - it's a great product).
These things may sound silly, but they add up very quickly to being more productive - especially if like me, you develop for traditional Windows and for non Microsoft mobile apps, rather than concentrating on one or the other.
I'm of course coming at this from a company perspective, where $1,000 isn't a number that is a showstopper. If I were doing this as a hobby or as a small independent developer shop, then I would probably go for the cheaper, non visual studio approach.
I'm just installing the bits now and in my case, I'm going to run it all on the mac, rather than have a separate mac and pc setup. I run Windows 8 using parallels and so using Visual studio inside there is apparently supported. I'll let you know how it goes!