Search marketing harvests demand, it doesn't create it (ht to Drew at Dropbox).
Most small businesses believe that they're too small to have an impact on the whole market, so they resort to picking the fruit that's already grown instead of planting their own seeds. It's far easier to wait until someone is ready to buy than it is to persuade them to buy.
Except the answer isn't to poach demand at the last minute. The answer is to redefine the market into something much smaller and more manageable. You don't need to persuade everyone that you have a great idea, you merely need to persuade one person. And then make it easy for that person to share.
One last semi-related thought: Wenda Millard quotes a Mercedes Benz exec, "If the only time I show you a Mercedes ad is just before you're about to buy a fancy car, I've lost." The fact is, advertising to build brand and recognition and demand is a very long-term proposition, not something you measure with clicks.
A last-minute swipe of purchase intent is a tactical win. It's not, however, a long-term way to build your organization.