Jesse,
Your question was similar but about sub sites, with sub sites at least if using different host names for the sites they would have independent logins and content and no malicious script from one can get to the other so there is not so much risk there. There is some potential risk with folder based sub sites because cookies are per domain not per folder and if someone in a folder sub site adds malicious javascript he can potentially steal cookies from any of the sites if the user visits his sub site.
You can create a sub tree within a site that a user can edit as bprado is talking about doing and you can give a user a folder sub site and it will be fine if they are trusted users but if you are letting anyone sign up then you need much more control over what they can and can't do and you don't want them to have the power provided in the mojoPortal content features because as I said these features allow the user to do things that could be bad in order to allow them to also do cool good things by letting them put javascript in the page.
mojoPortal is not a framework for building social networking sites, the security issues in a social networking site are much different and that is not currently a goal for the project. mojoPortal aims to provide what is needed for a business web site with social features to leverage other social networking sites for promotion but not to build the next facebook.
Best,
Joe