I'm just getting started with Logic. I've got a new computer and have added a 480 MB solid state drive. The SS drive is faster by a long shot. Where's the best place to store the instruments and effects plugins? What about the song files?
I just left the Logic Pro samples and data on the SSD and put all other things on a second drive (in my laptops case, a second SSD).
One thing to consider: If you use a template with a number of sample components, you might want to locate them on your main SSD as well. That way your template loads VERY fast. I've done with for a client who uses a few Kontakt and custom EXS libraries in his template: they all live in the "shared" user folder.
So, I suppose the best answer is this: depends on how large your extra libraries are and how you use them will depend on placing them on your SSD or not.