Hi
Though router posses some of the firewall features with the latest ios codes available also the firewall which has limited capability of doing or running routing protocols it depends on the topology where you are deploying them up.
If you need more and more routing functionalities like running multiple igp there comes the router which is specifically desinged for serving that purpose.There are several other features which comes by default in router or firewall which may not be available in them..
on the other hand if you consider the security measure being taken in the corporate world you just can keep the router in the edge to take care of both routing as well as firewalling feature.that will unecessarily load up the router .
So in this case the standalone/seperate firewall box will be the best bet to take care of the security part.
if you have a SMB kinda scenario then you can think of putting both firewalling and routing functionality in single router and configure it up accordingly.
in the same scenario you can avoid using a router and make use of single firewall itself to take care of simple static routing and firewalling integrated on the same box.
In some other scenarios you got to have both router facing the public world (untrusted network) and your local lan behind your firewall.
It basically altogether depends on the kinda traffic,amount of traffic,your requirement and the topology which you desire to have in your network.
regds
