I can give an example of a benefit I received. I was living in China and needed a VPN to more easily get to the outside world or use USA services like Hulu Plus. Best option is to have the router handle the VPN. Astrill, the VPN service I used, had a good add-on for DD-WRT that made the whole process painless.
I have an ASUS router now that I am back in the USA and the stock firmware is not only fine, I makes some things like restricting access for my kids much easier than DD-WRT does (DD-WRT can do I, but the process is not as simple).
Michael