It seems be impossible to do the following easilly with the current userlist dialog:
1) Make the user list visible
2) Visually track through the user list
3) Type selected nick's in the text entry box.

This can present problems in situations where a user/op wishes to address a message to several users in the user list without keeping a mental list of users present in the channel. Typing a nick with an awkward spelling can be nearly impossible without resorting to nick completion.

It also presents difficulties when typing confidential messages, as it becomes awkward to know who might be listening. A message intended for a small audience can easilly be accidentally sent while someone you forgot was there is listening. The only visual indicator of a channel's population is the enter/leave notices in the text area, which may well have been scrolled away hours ago.