Apparently, the current behavior for the Sales Insight Outlook plugin is to create SFDC email templates from every email sent via "Send and Track". The solution recommended by Marketo Support is to simply turn off "Sales email sent" activity in the SFDC sync, but I think that's a terrible compromise.
1. I now have hundreds and hundreds of templates in SFDC to sort through which aren't actually usable templates. 2. I've lost the ability to view the content of any "sales email sent" activities within Marketo.
As I can't think of any scenario where I'd want one-off sales emails creating new templates in SFDC, I think the behavior should just be changed. If nothing else, there should be an option to turn off template creation.