As far as I'm concerned there are no cons to opening up your server to the public, so long as you take the necessary precautions to protect it (ie. lock down the box as you would any publically accessible server).
If your goal is to run a public XMPP server, then you may want to take a peek at the very nice Registration and Client Control plugins (found inside Openfire admin interface --> Plugins --> Available Plugins). Registration plugin will create a web portal for users to create accounts on (you can edit the html i think to say whatever you want), as auto-assign new users to a group (like a general group or whatever). The Client Control plugin will allow you to provide a nice easy way to download the latest Spark IM Client or whatever XMPP client you prefer, as well as control some other user action stuff.
For a CA signed SSL cert, google "Godaddy SSL" and the first few links will bring up their discounted SSL certs (going for $6 right now I think). It should work for your purposes.