There is a valid point, if someone wants to transfer £150,000 or so for a slightly dubious transaction,you are not going to do it via electronic transfer as it leaves a trail, so you get the wads of 50's out and just tell the bank ''Yes, I sold my Rolls Royce for cash'' cash is untraceable.
A point in favour of £50 notes.Electronic or wire transfers are expensive.
£150,000 for example would cost a few £ hundred in fees , cash transfers are free !