If you bought a house and it appraised for less than your purchase price, how would you feel? Many times a lower than purchase price appraisal will kill a sale. But what if it didn’t? Would you ask the seller to renegotiate the purchase price? An appraisal clause could give a buyer the right to renegotiate the price or walk away from the sale if the appraiser felt the buyer was overpaying.
As a buyer in today’s market, this would be of consideration to me. But if I were happy with the purchase price (maybe satisfied is a better word) I would probably not make this request.
After all, price is relative.
Mike Elliott's Blog 
An appraisal clause is essential in this market. If it doesn’t appraise, you have the chance to renegotiate. I did this last spring with one of my clients.
BTW- This is a standard clause in our board contracts, so we’d have to cross it out to eliminate this contingency.