I don't know the actual statistics, but I believe many people decided to upgrade their old XP systems to Mac as opposed to Vista or Win7 (I'm one of them). Since I still have one foot in XP for legacy reasons (via VM) and also have a separate desktop with XP, I would really appreciate if Apple could seamlessly bridge my worlds by providing XP support. So you got my vote...
The installed base of active XP users must still be huge, and considering that Microsoft's support for XP is waning, I would think that anything that Apple could do to help provide a bridge to other XP users would be good business.
Right now, I use Chrome as my browser, so sync'ing bookmarks cross-platform is a snap. I was hoping Safari/iCloud could take over this functionality, but no, not for the XP side.
Since my company has hosted Exchange (one of Microsoft's cloud offerings), email sync between Mac and XP is handled by virtue of MS Office being installed on both systems. Also, it was very easy to connect my iPhone to the Exchange server too. So mail is taken care of outside of Apple/iCloud here too. It all works, but I would not call it seamless.
For cross-platform file sync, it is hard to beat Dropbox, at least for 2GB worth of storage. Here again is another lost opportunity for Apple/iCloud. I heard that Apple tried to acquire Dropbox, but since that didn't happen, it may be that Apple comes up with something equal or better to it...but here again, if XP support is not an option, I will have to stay with my collection of parts to provide the bridges.