Overscan is part of the broadcast SD spec, IIRC. There's a certain amount of leeway afforded analog transmissions because of the distortion that typically occurs on the edges--especially the topmost lines. Some TV's that you will see--typically those in stores--will also be misconfigured by their owners.
Overscan is also used because the current market is a mess. Every "HD" device supports (physically and/or virtually) different parts of the "HD spec," and the support is shoddy at best. When the typical (ignorant) consumer brings home a Sony HD set and finds that his DVD player no longer works properly (i.e. aspect ratios, overscan), what does he blame? The new HD set. Overscan is a catch-all solution that satisifies the average consumer ATM.
I support aspect-ratio-correct scaling and would love to live in a perfect world where all the devices on the Best Buy shelves automatically worked in any feasible combination with optimal scaling and letterboxing relative to the individual devices' capabilities, but that's just a fantasy for now.