From an organizational stand point, this is a really useful features that most browsers have (and tabbed text editors and other places where you have to manage tabs). In addition to Ctrl+PageUp/Ctrl+PageDown switching tabs, using Ctrl+Shift+PageUp/Ctrl+Shift+PageDown is a common way to move tabs around.
The patch to make pcmanfm do this is really easy, as it's just using GtkNotebook, which supports this already. I have attached it. I couldn't find any information on how to submit patches. I'm sure it's there somewhere but I wasn't seeing it. So I just took at stab at it. This is off of the latest git repo master branch for pcmanfm as of 5/30/2014.