Resist that urge to laugh, at least for a moment.

Pros: Customize seat height after a bike change without having to find a team car. Maybe put these on the Mavic yellow neutral bikes to start.
Drop a little for some rough stuff
Drop a lot for a high speed descent - safer than doing a tuck while crushing nuts on the top tube.

Cons: complexity and reliability (but these are getting worked out as the manufacturers gain experience with mtbs.

I would list weight as a con but with the UCI weight limit there is room for the added weight without increasing the overall weight of the bike.
They also come with internal routing, so no aero penalty other than a round seatpost.

(In case you're not familiar with dropper posts, they provide real time, on-the-fly, infinitely adjustable seat height over a 4 or 5" range. Simple to use.  Just push a button or pull a lever mounted on the bars and the seat drops until the lever is released, then holds at the selected height. Push or pull again with weight off the seat, and it rises until you release the lever. Super handy for technical off road riding; wins over skeptics, including me, very quickly. Reliability has been a problem but is improving. And now Thomson makes one. )