First of all, Kerry did not flip-flop - the much publicised issue was one where he voted against one funding bill, but because he disagreed with some of the terms in it. Meanwhile he voted in favour of another bill. That's how Congress works - you don't voted for one single issue at a time, you vote for a whole set (remember the "paper clip" Simpsons episode?).
As to this topic, I must say, I'd be asking the complete opposite: When someone changes their opinion when presented with evidence or argument, that is something to be respected! All too often, on forums like these it's just the same old debates with the same people sticking to their same opinions. That's normal, and it's hardly something difficult to do. When someone is convinced to change their mind, that's a very rare and wonderful moment.