PCManfm IS default in LXDE - it is what LXDE is built around. Kind of the corner-stone.
Yes - there is nothing keeping anyone from using the respective components individually, given its independent structure; you may use any component which constitutes LXDE, on any system - lxpanel, pcmanfm, lxsession, lxtask, lxrandr etc. It is kind of the fundamental idea of the lxde-team, as I have understood it. The components don't depend on each other. And here is the key - LXDE is a meta-package constituting of a base system, and some recommended packages (hal, gdm). If you like, you may make your own system any of these packages, but lxde needs the depend packages - as pcmanfm.
If you really like another file-manager - to answer your OP - you just simple install it - via your package-manager - depending on your distro. If you wan't to use gworkspace, just apt-get, and remove the speed-links to pcmanfm (the one on your panel and you may hide the ones in your menu) - and if you don't want PCManfm to handle the desktop - remove the the deamon at startup (you need to install another desktop/background handler) - but if you want to keep lxde, you need pcmanfm installed as it has dependencies to lxde-core. To set another file-manager as default (!!) - right-click on lxpanel->panel settings->advanced.
My first comment was sarcastic - I know. But I believe, if you really want to keep using lxde, pcmanfm is a great companion, and will always be there note: I am not a lxde-developer, and have no current affiliation with the lxde-team. The opinions here are strictly my own.