I'll let you in on a little secret: The default watch face does not run on the main processor, but instead runs on a much lower power micro which doesn't have enough memory to run Connect IQ. It honestly doesn't have enough memory to hold the screen buffer; I work with some mad/crazy embedded hackers here at Garmin. All that for a second hand.
We face similar challenges to other smart watches when it comes to balancing functionality and power usage. I think all smart watches have been figuring out how to offer the customize-ability of custom watch faces without killing the battery for the use cases the devices is intended before. We're starting with a conservative approach, but it could evolve over time.
-Alpha Monkey