Wow, a 4-year thread necro? Start praying to whatever elder gods are still out there, for the dead are rising!
But yeah, with lightweight engineering, there's really no
need to leave the heatsink behind anymore. You may never need to use it, but there are circumstances where it's helpful. Try landing on Skardee 1. Unless you're running a very cool ship, you'll probably overheat significantly before you get into glide. Getting back out is often worse.
Like a fire extinguisher, it's not something you will use every day, but it can be nice to have when the situation arises.
EDIT: I engineered a couple of them back in 3303, and have been using the same ones ever since. I almost never actually fire a heatsink anymore, and rather rig the ship to run cool such that overheating is pretty unlikely.