Never say yes unless you really want to marry the person.
There are two basic reasons why someone would propose in such a manner.
1. They want to catch you completely off guard and they made special arrangements to pull off what they believe will be a "special memory" that you will share for a lifetime. Hollywood has to bare some of the blame. Dinner for two doesn't cut it anymore.
2. They're uncertain if you will say yes. Therefore they want the crowd or audience on (their side) when you give your decision. They know most people will be rooting for a "yes" It takes a strong person to hold their ground and say "no" with what may feel like the whole world watching.
I saw one woman run away. It was funny. The truth is most guys already know if the woman they're with wants to marry them! Odds are she has "dropped hints", had discussions about raising a family some day, professed "eternal love", or other such talk has taken place before. Only an idiot would propose knowing his odds are 50/50 or possibly worse that she would say "yes".