I wasn't talking about performance, but about reliability. A PC should fail less often and e easier/faster to fix when it does fail.
Then again, I dislike throwing things away, so I would rather use the old PC than buy the router and later have to repair it if it breaks. Also, to use that PC I did not have to go to the store or order it and wait a few days for delivery.
 
I do not know how good/bad the flash memory inside the thing is, but I suspect that if it is used as designed (as a router), it does not write to the memory very often. OTOH, if you flash Linux to it and it happens to run out of RAM, it will swap and do lots of writes to the flash (or just start killing processes/crash if there is not enough memory available and swap is disabled) which could cause the flash memory to fail early. Also, if it gets corrupted, you can't boot from a CD and run fsck on it.