In certain cases, our subscribers do not use unique email addresses (in cases of family members who share email addresses) - so we have addressed this by assigning our numeric 'account id' to the Exacttarget 'subscriber key'.
Additionally, for 'leads' we do not have an 'account id' because they are not yet members, so we have assigned their email address as their 'subscriber key'.
My first question - how can I segment my subscribers so I can quantify how many have a numeric (account id) subscriber key versus a text (email address) subscriber key? I'm looking at Data Filters and groups, but it doesn't seem possible to segment by subscriber key, nor to filter by a data type query.
Secondly - what are the downsides to having inconsistent subscriber keys? Is it worth the effort to come up with a solution to make all my keys use a consistent data type?