For me, it's a cold weather thing - or really, a too cold for shorts but too warm for winter tights. I was going to buy knickers, but then I realized that probably wouldn't make me happy since I could start in knickers temperature but as the day warmed up I'd want to get rid of that layer. So instead of knickers I use Pearl Izumi knee warmers. They are comfortable and easy to shed...
(Just as background, to answer your question about type of bike - I only ride road bikes, skinny tired road bike, and slightly fatter tired touring bikes.)
--- Denise