DO NOT DO IT!
I was thinking about the same thing but then I realised that it would be just stupid.
The thing is that the iCloud ID is now associated with so many things apart from email and iTunes/AppStore password that I wouldn't want to share it with my wife for any purchases whatsoever.
So, what I understand is that I can keep making purchases on my Gmail account and use iCloud for all the other stuff; whether its Mail/Contacts/Calendar/Documents/Backup/PhotoStream/Sharing, etc.
This gives me the freedom to use my own content personally without anyone having to disable those options in settings.
Apple may introduce a feature in the future to share one's own PhotoStream with another iCloud account; so that could work.
But for now, its in the best interest to keep different accounts for Purchases and Personal Content; works for me.