I would expect that most people close multiple tabs with [cmd|ctrl]-w
If you think most people use keyboard shortcuts, we're working with completely different customers ;) Finding someone that uses the keyboard to copy/paste is decently common, but I rarely see use of shortcuts beyond that.
I find I use mouse close most often when I'm "info dumping", as mouse scroll and cmd+click to open a new tab are, for me, mouse operations. Rapid open/close of several pages is normal. Once I have 3-5 on a topic, I go "info diving" and nothing gets closed 'till the end, at which point cmd+w is how I tend to close out of the extra pages I had open. But cmd+w by even power users is, in my experience rare.