In the movie version of The Martian, Mark Watney pre-blows the escape hatches and removes the heat shield from the Aries IV MAV. What were the benefits of doing this compared to simply leaving the heat shield in place and blowing the hatches when he was outside of the Martian atmosphere? It seems to me like these would increase wind resistance and also greatly add to the chances that something goes wrong (debris whipping around the cabin and cracking his helmet, for example) while only producing a negligible decrease in mass.
"We'll remove the nose airlock, the windows, and Hull Panel Nineteen"
"You're taking the front of the ship off?"
"Sure, the nose airlock alone is four hundred kilograms. The windows are pretty damn heavy too. And they're connected by Hull Panel Nineteen, so may as well take that too."
As seen here. And in addition to these large components he removed everything else that wasn't nailed down and much of the stuff that was (e.g., extra chairs for crew, life support, backup control systems, etc. etc. etc.).
The book (and movie too, I thought) make it clear that without removing a metric shitload of mass from the MAV there was no chance at all of making a rendezvous with the Hermes. They also had to add more fuel.
The cost of putting mass into orbit (or in this case, rendezvous) is high, see, e.g., here. You can't wait until you get to a high altitude before jettisoning mass - you've got to get rid of it before you start - because the rockets have to push it all the way up.
(Update: A major part of the problem was that the MAV was designed to take the entire crew up to Low Mars Orbit where the Hermes would be waiting for them. But for Mark, the Hermes was on a fly-by and the MAV had to get to Mars escape velocity ... much much faster.)