The answer is no. Here's why.
REASON #1:
It's not respectful of my current clients. You wouldn't want strangers dropping in on YOUR wedding, would you? Nor would you want the DJ's attention at YOUR wedding diverted by his interest in a visitor to whom he's trying to sell his services.
REASON #2:
It's not actually that helpful to prospective clients.
If you dropped by just for a few minutes, what would you learn? If you came by during dancing, you wouldn't observe how I conduct myself making announcements. If you came during dinner/background music, you wouldn't get a sense of the dancing. Yet if you stayed long enough to observe me in action during different phases of the reception, you'd be substantially intruding on someone else's private event (see Reason #1).
Besides, since I customize my services to fit the tastes and desires of each wedding couple, what I say or don't say, play or don't play, do or don't do at YOUR wedding will differ from what I say, play, or do at someone else's wedding. You might walk in on my other wedding just as I play a song you dislike -- a song which I'm playing because THAT wedding client requested it.