It’s certainly needed, as Jeff Sadow points out.
Imagine if Hillary Clinton could have changed her vote for the Iraq war to ensure that it would happen, then could change her vote to be able to campaign that she voted against it.
It gives a whole new meaning to “I voted for it before I voted against it.”
Requiring complete consistency for politicians (or anyone else) is unreasonable. Anybody should be able to change their mind, but I expect a legislator to have thought about the matter up for vote and to vote honestly as MY representative, not as a representative of their future personal gains, such as re-election. I’d like to think they thought about carefully enough not to change their mind 8 hours (or less) later.