You could use either one for tables that have both, possibly there is some minute performance advantage to the integer ids.

The guids serve several purposes. They make it possible to join different tables to a single related table. So for example if I build a tagging system I can tag content and I can tag users using the same tagging system. For users I join by the userguid to the tagging tables to retrieve the user tags, for other content I can retrieve tags also by guids, so I don't have to build a separate tagging system for user and content, I can build one system and use it for anything that has guids.

The other benefit of guids is to make it easier if I ever need to somehow combine the data from 2 or more mojoportal sites into one site. So if one company buys another and they want to combine their users and sites it is possible. Imported rows will get new integer ids, but it will be possible to write queries to restore the correct relationships based on the guids.

Hope it helps,

Joe