Why do I need a firewall?

["Mullen, Matt" <Matt.Mullen () globaldigitalmedia com>]

I have several remote sites that will be Internet connected and I am
contemplating putting a dedicated firewall in each one of these locations.
I am somewhat of a beginner at this,  and I am trying to find justification
for the dedicated firewall as opposed to using the externally connected
router to filter traffic.  None of the remote sites will have any systems on
the inside that will need to be accessed from the outside,  no web servers,
smtp, etc.   Couldn't I get away with running NAT on the router with one
global IP address on the outside Internet connection, private non-routable
IP addresses on the inside,  and then lock down the router further with
access lists?  Wouldn't this provide adequate security to keep intruders
from the Internet out?   What would be the vulnerabilities in implementing
this type of scenario?  

Thanks in advance,

Matt Mullen 
Sr. Network Admin
GlobalDigitalMedia.com