Advantage of Filtering in Router vs Firewall

[Bill Husler <bhusler @ community . net>]

I know I have read here that You have a stronger, more secure, setup if 
you keep IP filtering in your router and use the Firewall for Application 
level stuff - like proxies, but I don't remember the reasoning behind it. 
Please, somebody, run through it again for me.
Thanks, Bill

The opinions expressed here-in are my own. Any similarities between these 
opinions and those of any other person - living or not - including my 
employer are purely coincidental.