Having been outraged initially by pointy ears in the films, I was persuaded by that quote in HoME. I don't think that the genetic variation dictating pointy ears would prevent interbreeding any more that the one affecting eye-shape does. I mean of all the differences between elves and men, ear-shape would be the least significant surely?
The other possibility is that since ears (and nose-tips) are made of cartilidge, they do actually continue growing throughout life ( bad luck Christopher Ecclestone
)... so maybe the extensions merely reflect their long elvish life
.... they could all look like Dumbo by the end of Arda...