Yes, any distro can certainly be configured as a firewall.

The primary advantage I see in using a "firewall distro" is that it's 
just plain easier, meaning that you don't need as high a level of 
expertise in order to make it work. Personally, I'd rather let the 
people who do IPCop worry about the intricacies of iptables and other 
aspects of firewall functionality (i.e. VPNs). There's also less of a 
chance of me screwing something up that way. Using a tailored distro is 
a way to leverage OPK (Other People's Knowledge). Why re-invent the 
wheel? Then again, if you're comfortable and proficient with iptables 
and OpenVPN (or IPSec or whatever), I expect you'd be perfectly 
comfortable "rolling your own".