Okay, I was going to post an intelligent summary of my views and how I feel things should be handled, but BardStephenFox did it better.
I would still like to add that attacking without letting the villain rant isn't neccessarily bad roleplaying; often it would be quite out of character for the characters to just stand there doing nothing while the villain talks. Yes, I know, that can spoil the dramatic moment...but I like to have characters respond realistically, not in a manner that begs heckling. That doesn't mean that I don't like villains who rant; just give the characters some incentive to hear them out. From another perspective, it also becomes more logical for villains to indulge in rants if characters don't always immediately attack. As BardStephenFox pointed out, characters may be less trigger-happy with their crossbows if they realize the villain might have information they need. Failing that, there are always spells for villains to use (i.e. Wall of Force, possibly illusion spells) or situations for DMs to use (characters can't see the villain, characters are captured/helpless or pretending to be captured/helpless) that prevent the characters from attacking.
Of course, sometimes your plans can go awry. I once planned to have an evil wizard cast
wall of force on the first round of an encounter, taunt the heroes from the other side of the wall, and then teleport away. (Fortunately for the characters, all the rest of his spells had been spent gaining the magic item he was taking away with him.) The party won the initiative, cast
silence, used the wizard's familiar to interfere with the wizard taking the magic item... long story short, the level 4 party killed the 9th level evil wizard. I could have fudged things, but I decided the players had earned the victory and I could adapt the plot. The moral of the story, however, is have the preventative measure in place BEFORE you roll initiative...