When I got my top case I thought I would use it primarily for longer tours (mainly for "aesthetic" reasons like BkerChuck alludes to), but in practice it stays on almost all the time. When touring it carries the stuff I may need on short notice on the road - mainly liners/layers to deal with the wide temperature swings we see in these parts (and hats, shorts, shoes, etc.). For day trips/short runs/solo rides, it's way more convenient to use my Givi remote opener to pop the lid and throw in a couple of helmets and gloves when I park than it is to use the key to open 2 side cases and stuff helmets in there (with one wanting to slide out on the "downhill" side).
Another factor is my running/brake lights on the case. Any time I'm riding at night I much prefer to have the case on because it adds about 200% to the area/intensity of the tail light. Bottom line, it's too useful and easy to use to take it off, and I'm at the point in my life where looking like anything less than a magazine cover model isn't my biggest concern....so it stays on.