Java is a dirty word in most user households. We've all had experiences with the bloated, flickering, slower than molasses results of a poorly coded applet. One example of better programming through Java would be the lovely eForum. For software and service related sites, the varying degrees of public and private forums are ideal for offering premium service to paying customers. With today's telecommuting and remote teams, this is the best way to manage projects or keep in touch with far-flung workers.
Security is always paramount to any operation in today's Internet.
You're the first and last word as to what sort of content will appear on your site. Any damage they might do is limited to the power that you give them. Expresso also allows you to edit user classes to include or exclude them from certain conferences.