Here is a little more meat...
A Security group has all the features of a distribution group, like sending an email, and more - They determine access rights. The issue is a security group is used in the logon process where a distribution group is not. What this means is that the more security groups used as distribution groups could slow the logon process down for user. The Access Token that is created by the logon process contains the security group membership, the are no distribution groups listed in the Access token(which is stored in RAM on the users PC) because distribution groups do not determine access.