Not sure who downvoted that. I've seen lots of Apple folks carrying these expensive DP to HDMI or VGA dongles when Apple went DP-only before DP even became common. Think Different, I suppose.
Define expensive, I guess. At my work and in my personal life, always just ordered whatever display port adapters are on sale at various tech stores. Even early on in the era of Apple with mini display ports, the form factor attainable via the mini display port is worth the extra $30 (from apple) or $10 (from else where).
So yeah, I guess there is value in spending $10-$30 for an adapter you use as occasionally as you might use a full DVI or HDMI port.
HDMI is not much bigger than mDP and doesn't add thickness over USB ports which already exist on those laptops. OTOH, it was more commonly supported by non-Apple monitors in that time and very easily adapted to the ubiquitous DVI-D (same protocol and cabling, different plug).
If Apple cared about interoperability and user convenience (no silly dongles, yay) they would have used that.