You might outnumber them, but you may be less vital. A person that takes the time to write is a potential evangelist.
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.
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.