Most of the time you will not use the command line to start the client. However it is handy to remember -h and -p options, so you will be able to connect to a server even when something is wrong with the metaserver (which happens from time to time). Or, for instance, if the server you want to connect to doesn't report itself to metaserver (so it won't appear in the metaserver window).
It is good to remember that default ports for most of servers are 2592 for players and 2593 for observers. So if you want to observe game on continuum.us.netrek.org you will use "netrek -h continuum.us.netrek.org -p 2593".
Here is the list of command options for NetrekXP:
-h <server> : specify server name to connect to -p <port> : specify port number to connect to -r <configuration file> : specify configuration file to read -s <socket port> : specify socket port -u : show usage information -A <password> : specify character password -C <name> : specify character name -U <udp port> : specify UDP port -o : use old style binary verification -R : use RSA binary verification -P : log server packets -c : check players on server and return list -f <file name> : record game to file (can be viewed later with -F) -F <file name> : view recorded game -l <file name> : record all messages to log file -m : connect to metaserver (default) -k : display known servers -n : show console window -v : display client version info