The main reason Fivers (and other notes) are grubby and scruffy is down to the modern trend of not using wallets any more. People just screw them up and put them in their pockets.
£5 coin - I think not. The £2 coin although around isn't very popular and not often seen. I can't remember the last time I had one. And just think if Supermarket trolleys were to take them, firstly it would be hard to find one, and secondly I've lost enough pound coins when they've dropped out of the trolley mechanism, that now I buy trolley tokens for about 30p each as its cheaper to lose these, and so certainly wouldn't want to lose £5 coins!