The default interface for the auction house could use some loving. It's an essential part of players' gaming experience, and whether they use it or not, its design has an effect on them. We've seen many examples of Blizzard designing parts of their UI to include features provided by addons that had become so prevalent that everyone had to run them. For example, threat didn't always show up on the default frames -- back in my day, we had to run Omen to avoid pulling aggro from the tank.
It's important to recognize that not every valuable interface tweak belongs in the basic WoW UI, though. If it's a feature that's only used by a small segment of players or is unnecessarily complex, it's probably best off in an addon. You won't see me advocating that the base AH UI keep historic pricing numbers, for example. It is, though, a travesty that you have to page through hundreds of pages of single auctions to get to the lower priced stacks.