Lot of networks have only access to http/https services, in addition to this you get (for free) all of the authentication mechanisms supported by webservers (like kerberos, radius, ldap and so on ...)
at this point i have no idea of what an ssl vpn is :). The two kind of them i am aware of are openvpn-like (a custom service running on a tcp/udp port, but not on http) or the ones that are more "proxies" to internal networks. They do not allow to interconnect peers or to use services like bonjour or any other broadcast/multicast technology as they are at an higher layer. Am i missing something ?