The complexity of the daemon is not really the issue. The frequency of use is. xinetd allows you to get increased logging, and depending on the configuration an extra layer of security. All of the daemons you mentioned can be and are run under a super server in some configurations.
If you rarely use the daemon consider running it under the super server. This will allow you to reduce the number of daemons running and the resulting overhead.
inetd and xinetd have some very simple services built in. These services should be disabled unless needed. Other services don't have a daemon interface, so must be run by a super server. Many services which have network daemon capabilities, have flags which tell them they are running under inetd.
You should consider startup overhead and configuration stability before running daemons under a super server. A heavy startup overhead is one reason to run a rarely used service as a daemon. The services you listed are usually relatively heavily used when installed, so are run as daemons.
Apache is optimized to run as a daemon. If it is used much it is best run as a deamon.
Mail servers generally fork a new server for each incoming call, which has low overhead on most OSs. If it isn't used as a mail hub or delivery from remote sites it shouldn't be listening on any address other than localhost. It doesn't need to do that unless you have programs running on the server which use TCP/IP to send email via localhost.
sshd has relatively low startup overhead, except for random data for encryption. It does require a fair bit of random data, which may not be readily available if run under a super server.