When recruiting and holding interviews, it's always a two-way sell. The candidate is selling why he or she would be a perfect candidate for you, and you are selling why your company would be their dream employer. If you think one step ahead, a candidate's perception of you and your company when he or she leaves the interview can (and will) have a far-reaching impact on your business and recruitment in the future.
If a candidate walks away thinking what a great company you are and what a dream it would be to work for you even when he didn't get the job, then you've done your job as an interviewer. If, on the other hand, he leaves the interview thinking that you were a jerk, that he didn't get a fair chance to prove himself or just plain insulted that you ended the interview prematurely, then that's a poisoned pool for you now. He or she will tell everyone they know that you're a bad company and any potential interactions you have in the future will be biased against you.
There is no room for personal pride when recruiting, it is never appropriate to give anything but your best and most compelling performance when interviewing and recruitment is about much, much more than just the interview. We drill this into all people at our company that are involved in interviewing new recruits and it pays off. We have numerous instances of people who failed to qualify but who left the interview feeling so good about our company that they still recommended us to their friends, leading to some really great hires down the line for us.