It does seem fairly obvious that there is more vandalism and hooliganism on
buses without a conductor. This does not mean that conductors are allways able
to stop it - on late-night buses particularly conductors (and conductresses)
themselves have frequently been the unfortunate victims of violence, including
stabbings, by the fine upstanding young of London.
I remember once a clippie (there, showing my age!) saying she would use her
ticket machine(swung from a strap round her wrist) as a weapon if she had to,
even if it meant losing her job. Judging from the reported threat to a driver
who threw tea, it would undoubtedly mean her being penalised for defending
herself.
But quite often the trouble these days comes from quite young kids who can be
quelled if dealt with firmly by an adult. Unfortunately many adult travellers
today seem too cowed to do anything except stare intently out the window when
something is going on, and don't even resist when they themselves are
targetted.
But a conductor (when I was a kid they were sometimes also called "guards")
would instil more confidence in the public and remove some of the cockiness
from the yobs.
More positively, we would have fewer accidents such as old people (and kids)
being thrown to the floor, or small kids trapped in closing doors, if the
driver was not having to collect fares, then start up, and was not expected to
know what was going on in the bus while keeping his or her eyes on the road and
traffic. (I've seen people thrown down because the driver had to suddenly stop
because some motorist suddenly pulled across in front. At least if someone was
there to offer a steadying hand to the passenger...)
One last point, to those who have offered examples of conductors not doing
their job properly. LT used to employ not only uniformed 'Blakey' inspectors
but plainclothes ones who hopped on the bus to check on the conductor (and in
one case I know, even acted as a bit of a provocateur). There seemed to be
more regulations in those days too, like how many standing passengers could be
carried, and so forth.