Is there benefit to having a personal Mandrill or SendGrid account?
The main benefit is that you would be able send your own transactional emails through Magento, (e.g., shipping emails, confirmational emails, etc.), in addition to your MageMail emails.
However, if you setup Magento to use Mandrill and setup MageMail to use Mandrill then both triggered email and transactional email would be sent through the same provider and you could see stats across both of them in the same place.
The same would apply to SendGrid, as it behaves similarly.