What you need to remember is that Blizzard's retail servers are a corporate project. While we're close, MaNGOS is definitely not the exact same as Blizzard's servers, and of course isn't corporate with funding in the ten-thousands.
The 'caching' of data that the Blzzard's database does would be all okay, but the constant transmission of encrypted data when we could already have it on board the server would result in a massive bandwidth usage increase.
In a nutshell, if you're having issues with MySQL database sizes, you may want to look at purchasing a bigger hard-disk.