Actually it's less convenient to use the screw-in variant. You have to compose with the holder and the square filters, take off everything, screw on the ND110, screw on the holder again and shoot. For any adjustment you have to repeat this process again and again. With the Big Stopper you just slide in a filter and take it out afterwards. (I'm new to Lee but that's the best part of it). I think the 77mm B+W ND110 costs actually almost the same as a Big Stopper (in the EU). If you ever have the chance, go for it, you already have the system!
big stopper price almost double from B+W, 200 $ against 110$ here (in indonesia).
for some case (camera without live view) it's rather to use one system as you said, I'm totally agree with you. but in my case.. since I use a 5D MK II with live view mode it's help me to do anything with filter attached on Bulb mode. most of my photograph are long exposure photograph, so I never take off my B+W from my 17-40. it can said my UV filter is my ND filter
while GND I only use it not for all condition. I use it only for several condition such as I need another concept for the sky like this pict. if the contrast range between the sky and the land are acceptable (1 to 2 stop) I think reducing GND filter is better, based on "more filter more bad your photo quality".