Two instances, two listeners and /etc/services

A few weeks ago I created a second instance, recreated the tnsnames.ora, sqlnet.ora and listener.ora files with network manager (one listener on port 1526, the other on port 1527). I can now connect to both databases over the network. But I forgot to update my /etc/services file on the host machine.
According to Oracle7 Server Installation Guide for HP 9000 Series 700/800 Release 7.3, Prepare to install SQL*Net version 2 products, Setup for the Oracle TCP/IP adapter, I added this line when installing Oracle7:

listener 1526/tcp #SQL*Net version 2 listener port

From Oracle7 Administrator's Reference for UNIX Release 7.3: "Oracle recommends that you reserve a port for your SQL*Net listener in the /etc/services file" and "Reserve more than one port to start more than one listener".
So, I should add this line to /etc/services:

listener2 1527/tcp #SQL*Net version 2 listener port

The file /etc/services associates service names and aliases with the port number and protocol, says the man page.

I do not understand why exactly I have to add these lines to /etc/services.
What does Oracle mean by 'reserve a port'? Do I really need a line in the /etc/services file for every instance's listener to make it work?

I have Oracle7 Server 7.3.2.3 on HP-UX 10.20.

Thanks.

--
Ludo Joris
RUCA - University of Antwerp, Belgium
lujo_at_ruca.ua.ac.be