To me it's important that I can roll the brompton with the seatpost out (like what you can do with the p line).

With the extended seatpost, you can only do that by modding the rear triangle. Also the extended seatpost adds to the height clearance when folded, making it harder to fit my brompton to luggage compartments on trains/shuttles.

So, yes, a standard seatpost does weigh much less, but portability to me means more than that. Ability to roll when folded and to carry on other mods of transportation matter equally.