Some people learn from other's misfortunes/close calls, some either need the experience first hand, or live in denial ;D

I use a slipknot on anchor whenever it's deployed, but always have my knife handy.  When you see movies or read stories about the days of iron men in wooden ships, you understand that even though they were the masters of ropes and knots, they always had a sharp knife on their person. A knot is essentially a mechanical device. Like all devices, they can fail/foul at the most inopportune times.

A knife, assuming it's metallurgically sound only needs to be kept as sharp as possible to perform it's primary function. ;)