For some people it is a real problem... I get 100-300 emails a day and I know people who get a lot more. And these people aren't "well he's pg, of course everyone emails him for advice" these are just general day-to-day emails.
Personally I'm with you in valuing politeness over efficiency, but I do spend a lot of time reading and responding to emails, and sometimes I get a couple of days behind in doing so.
(Maybe my experience differs to that of many HN readers as I'm not a hacker, I work in advertising and publishing.)