That might be good advice if the customer contacts you one-on-one. But when the customer posts their complaint to a message board that's read by thousands of third parties, it's helpful to put up an explanation of why the system was designed the way it was. Otherwise I, who have never used the service and am just skimming this thread, might come away with the wrong impression.
It's important to avoid getting into a flamewar with your customers, but pointing out the rationale behind your design decisions isn't necessarily offensive. Yes, it's not what the complainer really wants to see, but it is what I, the silent reader of the thread, want to see. And people like me outnumber the original complainer N to 1.
And you'd better be pretty confident about 'N'. Threads like this and readers like yourself may provide a short term boost or bust, but communicative visitors can broadcast the message off-line for months to come.
I think you'd agree that we should aim higher (much, much higher), than just avoiding getting into a flame war.
You may have been confused about the scoring system using to calculate reputation. It's a little more fine-grained than a traditional up/down karma system. Since voting up gives you +10 rep, getting 15 rep is as easy as getting two people to vote up your submission.
1) The site builder added a certain effort treshhold to block out spammers.
2) The complaining user doesn't want to invest the effort to cross the treshold.
These statements are mutually exclusive. The only way to resolve this problem is to remove the need for a treshold. (i.e. remove spam from the internet)
Even if something isn't feasible, that doesn't make the customer wrong.
Complaining customers are opportunities to step away from the whiteboard and listen.
Customer: "I would like it better if every time I opened up your web page a slice of buttered toast came out of my CD drive."
Response: "[APPRECIATION: Thanks for your feedback]. [EMPATHY: We understand your desire for a tasty breakfast], and [PLAUSIBLE CONNECTION: we see how a website about coffee should help you plan a more complete meal]. Unfortunately, I'm not aware of a way to do this right now. Just for future reference, [OPPORTUNITY FOR MORE INVOLVEMENT: if we were able to do this, would you like to help us test, and would you be opposed to margarine]? For now, what if [SOMETHING CLOSE TO A SOLUTION: we began building a complementary breads chart, like [example]]?