PC based routers are useful.
The CPU's in modern PC's have massive grunt compared to 'proper' routers.
Granted routers do have ASIC's to take the load off the CPU somewhat but some tasks just eat up all the memory and hit the CPU hard.
Where they fall down though is interface support.
Copper ethernet and SC fibre gig is no problem but everything else with the exception of ADSL cards are difficult to obtain and very pricey.
Try getting POS cards for them.
Tried getting an OC48 card in PC world lately ?
What they are exceptionally good for is natting.
You can have hundreds of RFC 1918 clients behind one all hitting the Internet hard and they don't break sweat.
Load it up with ACL's and a FW for good measure and CPU load will still be low.
Another good use for a PC based router is BGP.
With the amount of memory you can have in a PC they have no problem holding full tables.