Might as well chuck in my reasons.
I have a MAC with options of OSX or OS9 for operating system.
I have a few OS9 and OSX extra plug-ins. Some are upgrades some are not.
My monster Wacom Digitizer II (tablet) is an older style serial connector, not a USB and only runs in OS9.
At one time I had Photoshop installed in each partition, with duplicate filters all over the place.
Being a thrifty soul, [bustagut] I bought a Belkin Serial > USB converter which meant I could run the Wacom, but only in OS9.
Installing just one Photoshop and standard set of plug-ins, I saved oodles of space. All extra plug-ins and filters are installed in a separate folder.
Now if I want to use the Wacom I just startup in OS9 and only the OS9 version plug-ins load.
If I don't need the Wacom, just the OSX filters load. :B
Al.