I wish stuff like this worked more often, but it's been documented to fail more times than it's been documented to succeed.
The main problem is that there are four different motivations for piracy, and the "ask them to stop" approach only works for one of them and does nothing to stop the distribution of the copies that are already out.
The first (and the one this works for) is the altruistic pirate. These guys believe that the reason software doesn't make any money is that it doesn't have enough exposure, so their goal is to drive adoption of the software. Unfortunately, the closest actual market you can look at to compare this to is the shareware market which has average conversion rates under 0.1%. The number of sales generated really won't offset the loss from units downloaded from people who don't share your motivation.
The second is the "Robin Hood" pirate. They steal from the (perceived) rich, give to the (perceived) poor. If they can make some extra money to help them survive in pricey neighborhoods, all the better.
The third is the angry pirate. The company in question made some sort of perceived slight towards them, so they're just going to crack and distribute the product to "punish" the company. The product could have been priced too high, the game may not have been long enough, the game may have not supported one of their video card features, the boxart may have been entirely the wrong color of blue, it doesn't matter. He's upset, so the company is going to pay because their customers won't have to.
Finally, it's the challenge pirate. These are the guys who crack the new protection methods, or find new ways to crack the previously uncrackable.
Unfortunately, "asking them to stop" won't stop the last three groups, and they're significantly more plentiful.
(Update: Fixed the typo. Thanks Sarkie.)