This is true when the seller is asking a buyer for a copy of their survey. It has value and the seller is not entitled to it for free, particulalrly when they would only care for it if the deal falls through.
On the other hand, if the seller already has one, it isn't a terrible place to start and the seller may want you to see it, if then think it will keep you from doing your own. Of course, you should have your own. A seller may then refuse to give it to you as it will simply become the baseline on which the next surveyor begins to look for discrepancies.
As a buyer, I always asked for a previous survey. I don't see this as an unreasonable request. Yes, it has value, but so does everything else on the boat you're trying to sell. I happily hand it over to serious buyers b/c I want them to know as much about the boat as possible.
As a seller, I try to anticipate all the questions a buyer might have, but as James Wilson says, I haven't anticipated them all. I also suffer from owner's-delusion: where we all think our boat is the prettiest, best, most wonderful boat out there (a fact witnessed every time people get into a "No! Mine is best"
fight here on SailNet). This is why I ask for, and offer, the survey.