I don't agree that brace heights are getting shorter. Generally most bows are hanging in around 7", which has been going on for many years. Sure, there are outright speed bows that have shorter braces. but these are just a few. There are lots of choices to be made with around 7".
The advantage to shorter? More speed, pure and simple.
Disadvantages? Usually a noiser bow. Wrist slap in many cases. Less forgivness in the bow because of the arrow being on the string longer for a given draw length.
That being said, a person with a shorter draw length can actually benefit from a shorter brace height. He can get more speed without having the arrow on the string as someone with a longer draw.
I'm sure there are a few bows with brace heights around 8" or so, but nobody is really interested in them these days. This is a different generation that is interested in speed and not so interested in accuracy potential.