TCP ping missing obvious port?

Just wondering if I'm missing something obvious, but is there
a reason why the tcp_ping() function within Nessus, used, for
example, in ping_host() to check if a host is up, will omit
port 80 as one of the default ports being checked using the
"built-in" TCP port list?

I note it's checking SSH, FTP, DNS, a secure web server,
but notably is missing any checks against port 80.

File reference: libnasl/nasl/nasl_packet_forgery.c
Function:       nasl_tcp_ping

Also, since the same function is strategically setting
the source port for any queries, (e.g. setting port 53
when querying a DNS server) to increase the likelihood
of a packet getting through, would it not make sense to
have either
   a) The script be able to set the source port, or
   b) Have the nasl_tcp_ping function use the same
      source port lookup technique for any user passed
      ports?

At issue is, for example, a user enhancing the 'built-in'
port list with one of their own, by, say, adding port 80,
but then losing functionality as a result of losing the
sport settings currently in use in the function.

Thomas
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