Using STUN servers does not reduce the security level of your network. The only real purpose it serves is to identify ports that are already open and can be used for signalling and and audio transmission. STUN servers are also used to collect information about your connection such as your public IP address.
So basically to sum things up, VoIP apps look for holes in the network that are already there but don't try to create ones. By default all routers and firewalls will already have ports open to allow for such communication (e.g. for VoIP, Games, Instant Messenging, PeerToPeer, etc.) and STUN servers simply aid in finding these holes.
If you block all the ports on your router/firewall except for port 80 (http web browsing), VoIP and other Internet based applications will either not work, or will have severely reduced performance (e.g. delayed audio, poor quality sound, etc.)
You can disable stun servers in Express Talk and manually enter the network details if you prefer, although this will not work well if you have a dynamic IP address.