The cost benefit analysis on Ethereal/etc vs Sniffer on anything
but the smallest of networks is usually very easy to make.
The fundamental issue is what questions do you have and
should you have about your network and what tool answers
those questions efficiently and reliably. Good protocol
analyzers sell because they save time in answering important
questions. Sniffer recently released a SMB Sniffer
called Netasyst...worth a look if cost has been an issue
in the past. So ends this biased response. 