A limited time offer can often move the skeptic and procrastinator from prospect to customer. Putting a drop dead date on an offer frequently will move that hesitant person to action. If there is only a short time to take advantage of a special opportunity often you will enjoy more response. People will do something vs. doing nothing.
In essence, a limited time offer will entice your prospect into action for fear of loosing this unique opportunity. You have to give them a reason to buy your product, and a time constraint could be the extra motivator you need to get your prospects over that decision-making hump.