Re: "why would you use your own?"
Dozens of reasons: your own control of firmware, no back-doors, ability to set up different wifi networks for guest and own use, custom firewall rules, maybe running a VPN end point in it, etc, etc.
Don't forget "pre-assigned IP numbers" (every router calls it something different). Yes, the Arris routers that Crapcast provides have the ability, but the interface is crap, and the router wants to reset itself for each and every number you put in. A decent DD-WRT or OpenWRT capable router has a CLI (SSH, telnet disabled) which can let you do this with batch commands.