Welcome to BeyondRelational and very interesting question.
Let us start with simple example. One server has two database. Database A and Database B. User A has access to only Database A and User B has access to only Database B. When User A is logged in he can do all the necessary based on permission in Database A but is logged in Database B as Guest. In default condition, he can not do anything as SQL Server does not have any permission on guest login (which can be found under ServerInstance >> Databases>> System Databases >> master>> Security >> Users >> Guest).
However, the feature is that if you modify the guest login and give any permission to that login (e.g. create, drop or anything which are applicable), automatically all the Guest logins (in our earlier case User A) will get those permissions.
I think this sometime can create compromising situation and all the other admins can overlook it (if one person has modified permission). For the same reason, you need to disable it. I think this is what exactly Jacob is suggesting in earlier thread.
commented on Jun 10 2010 6:26AM