This is a repost ( as requested by David ) so that this topic gets it's own thread.
There is a definite need to be able to selectively use ( or not use ) the option to share login registrations and E-mail addresses between sites in a WP Multisite install.
As it is right now, if an E-mail address was used on one site of the multi-site install, and a user attempts to register on another site with the same E-mail address, they get an error that it was already used and can not be used again.
I've done some Googling on it and it would appear that the E-mail address has to be unique for a password reset since WP is sharing the _users database table between sites.
While I can appreciate the convenience of this on a blogging network, it fairly renders WP Multisite useless in a commercial environment.
If a visitor registers on SiteA.com (one commercial entity - redirected from an external URL to MyMultiSite.com/sitea) and via some chance winds up at SiteB.com (another commercial entity - redirected from an external URL to MyMultiSite.com/siteb) , they should be able to use their same E-mail address again. Otherwise they will be wondering who has stolen their E-mail address or hacked their account.
So it looks like we need two things here:
1) A setting at the Network Admin level of the plugin to share the login registrations between sites ( or not
2) A modification to the _users table ( or perhaps use of an alternate table if need be ) that would use a two-part key ( the SiteID and the E-mail address ) instead of just the E-mail address to identify the login registration as a unique entry.
Having a plugin that will fix this ( or adding it to one such as Membership ) would be fantastic as this is currently a severe limitation to using WP Multisite in a commercial environment.