no, there's obviosly no system perfomance advantages sinse new process gets new resourses and take extra CPU time. Not all subsystems spawn new processes but only these that had been developed to. For example ftpd waits for connection it gets an connection and gives the way for a next connection. It does it by creating a new instance of itself while the old instance is occupied with an previos connection. If service is heavily depended on spawing there is limit in configuration for number of simultaniously working instances of one program. You can visit /etc/initd.conf or /etc/xinitd.conf and see the service "wait =no", or "respawn" for these services that will not wait for end of previos transaction to startnew one. By convension, all TCP services for example are "no wait". I learn with "UNIX Essentials and UNIX Core" DVD and with Emi Nemet book.
Hope it helps.