Good point, I'm sure some people would argue that "no" is also pretty powerful. We are faced with so many decisions every day that our ability to say no is pretty important.
http://www.realworldtech.com/forums/index.cfm?action=detail&...
"Yes" keeps things going as they are - because with every question comes an expectation of "Yes" - if the many single men afraid of approaching a woman are any indication, most frequently, when we ask a question we expect a positive response - the times when we expect a negative reaction, the questions don't get asked.
For that reason, "yes" is - in my opinion, which I feel could be substantiated by data - not nearly as powerful as the opposite, no.