I know we already have the ability to copy profiles from a sister site. However, from a usability perspective, if you want to keep your profiles in sync, it requires you to navigate to your profile/accounts page on each site and click that button.
It would be nice to have the button in the reverse direction. Probably, having a "Copy the current profile to all sites" button near "Clear all associations" is a nice place for it.
I just updated my profile on StackOverflow and you're right, having to go to each site to pull the profile across is a minor pain. It makes more sense to have a 'Copy To All' and perhaps change the individual copy buttons to 'Copy To [Site]' etc for more granularity if you want to keep one of the profiles different.