Lets say you had many thousands of users (as GT would well know I suppose), and each were lazy with removing old posts (like I am), I can see the size and performance of the database really suffering unless you have a dedicated server or multiple servers.
Setting limits on the number of messages may be benefitial to someone, I am not sure.
An example of the use of this feature, as a reminder next to the inbox and outbox (received and sent) messages portion of the user's Messages.
You have reached 27% of your inbox limit.
Incoming messages here...
You have reached 98% of your outbox limit.
Sent messages here...
If you could set a kB limit on the admin, then this would be possible, and it would just begin removing old messages when the limit is reached.
I suggested an autopurge function to Jason a short time ago( I think ), but I think this would be even better.
EDIT: Not comparing products, but I am used to this luxury with my existing forum software, something that would really be useful in GForum. See attaced pic...