When wiring our datacenter, buying bulk ethernet cables, you'll find a bad cable every 100 or so cables. Still beats the failure rate of making cables yourself.
On the other hand, if I'm wiring them myself, I can just cut the end off and try again, since I've got a whole pile of ends right there, and the equipment to do it. If I'm relying on the bought cables, it could be quite painful to re-run the wire.
