People have different length legs and torsos, this is the main thing to remember…
I have quite a long leg length for my height, so invariably when I ride a "correct" sized frame for my height, a 350mm seatpost is slightly too short. So I've become accustomed to just using 410mm Thomsons on everything as they're the one post I haven't managed to bend yet!
So to most people, my bikes look like they've got too much seatpost showing… But generally, they've also got relaively short stems on too. If someone is running a 410mm post AND a stem 20mm longer than ideal, they likely should be riding the next size bike up. But if like me you need the shorter bike, but with a long post cos of leg length, don't worry about what the nay sayers say… Have also seen plenty of bikes where you think about saying "put the seat up" to the rider as they've only got about 6" of post showing above the frame, but then you find out they've got ultra short legs and arms like an Orang-Utan!
