I think if one has the money to purchase a suit, by all means, do so. It is definitely a good idea to have a suit and a tie. Dress up nice and professionally is very important to make first impression.
However, it doesn't mean that you should dig deep in your pocket and spend a lot of money on a suit. I personally think that the adcom will judge you based on your performance, your answers, your attitudes, your confidence....etc. A suit is simply saying that you are a serious person, but I don't think it is the only factor that will make you a "rejecting" candidate. If you don't have a suit and the dental school adcom will not take you seriously, then I don't think i would go to that school anyway. Like someone has said, it is not a beauty contest. The adcom should know that most applicants do not have the money, especially after they have to pay for AADSAS, application fee, traveling expenses, etc...
I guess at least one should dress professionally, at least a black pant, a dressing shirt, a tie, with hair combed back and clean, and polished shoes are the minimal requirement. If you have a suit, you should wear it. If you don't, just dress up the best as you could. It is your attitude, your behavior, your answers, your confidence, and your respect of the adcom that counts the most....A suit is important, but I don't think it is the BIGGEST deal.