The infrastructure has to be there. As someone else has stated above, cash machines don't dish out coins. There's that along with the fact that none of the big three operators publish their single and return fares. Are people expected to carry pockets full of change around? Also, how does it stop people from paying for say a £4.80 fare in 5p coins? Would that not take the driver longer to count out? It's legal tender up to the value of £5 after all...

In addition to being transparent and publishing single/return fares, if there was a smart ticketing option that you can top up (like Oyster), and also the ability to make credit/debit card payments via NFC, then I wouldn't have too much of an issue with this. Until then though, there's no justification for moving to exact fare, and it would have to be a staged/phased approach at least. 

Out of interest Davey, can I ask what mode of payment you use for your bus journeys? I have a limited Arriva season ticket, but need to use cash for GNE buses, and I rarely have the exact fare to a T. Usually £3 for a £2.70 fare, £5 for a £3.20 fare... etc...