I in part, I disagree. Credibility is a factor in debate. If it is established that a pattern of dishonesty has been displayed in other debates, it can be grounds for dismissal of arguments in current debates
(in the interest of saving time) if proven.
On the other hand, it is also good debate tactics to put the opponent on the defensive. With a little skill, you can cause the opposition to unwittingly make your arguments for you...although, it's painful when they finally realize thats whats happened (some say it's dishonest, others say it's good debate skils/tactics).