I'm also kind of wondering about the choice of Thunar, since PCManFM is the default file manager in LXDE.Imagine someone would want it.
PCManFM can't do any file search, for instance. Only because of it I installed Gnome commander in my LXDE.
If we're going to let our imaginations run wild here, then we might possibly want to list every one of the 12,000+ applications in the PCLinuxOS repository ... imagining that maybe someone would want it.
My other method, of using the file selection dialog, IMHO, offers maximum flexibility for all the different autostart configurations there are likely to be.
EDIT (further explanation): There are bound to be as many ideas on what to autostart as there are users. I am aware that PCManFM fails miserably with file searches. But let's not forget that LXDE also comes with SearchMonkey already pre-installed ... I presume that's to overcome PCManFM's file search shortcomings.
If someone is savvy enough to properly install Thunar on their copy of LXDE, along with all of its Xfce dependencies, then they ought to also be savvy enough to be able to pick thunar out of a list of applications that are installed in /usr/bin.