True enough. We all know what political promises are worth. And politicians wonder why turnout is dire and they're so widely held in contempt.
An obvious example would be certain LibDems 'pledging' about tuition fees.
As it happens, I think they were hoist by their own petard on that one, thinking they could safely promise all sorts of cobblers for political expediency safe in the knowledge they'd never be required to fulfil it. But it shows what a politician's word is worth .... he'll keep it if and only if it's politically expedient. Otherwise, circumstances will have changed, or the "public interest" requires a pragmatic approach, or some other such excuse will surface. Or, of course, cabinet rotation will nullify it by them all having played musical chairs with their jobs and someone else will be in that particular hot-seat.
I remember seeing a cynical definition of an "honest politician" in a book once (Robert Heinlein, IIRC), and it went something like .... "an honest politician is one that stays bought". It's not that far off my own view of them.