It may not have an immediate effect on the environment, but if people are willing to do it for their own satisfaction then I've got no problem if they do so.
The irony being they hop straight into the Chelsea tractors to pick up Barnaby and Jonah from school...
The implication that it might cheapen petrol is silly, petrol makes the world go round, it will have the f**k taxed out of it whatever you do (see other thread for petrol debate).
but if everyone stops using them, I think it will have a bigger impact. so don't write it off, it can only help in the long run.