Keep it because it was your first real camera and if you sell it, one day you'll wish you hadn't.
Keep it because a backup - especially something as rugged and reliable as a Spotmatic - is always worth having.
Keep it because sometimes you'll be mid roll in your F5 and really want to do a few shots on another emulsion.
I have the 35mm f3.5, 55 f1.8 and 105 f2.8 for the pentax.
Keep it because those lenses, especially the 35mm and the 105mm, both of which are really good, have a very different look from that of the Nikon glass.
Keep it for the pleasure of photographing with something that (in 35mm terms at least) takes you back to the basics.
Keep it to pass on to your grandchildren.
Just my thoughts, but I think you can see that on balance I tend to feel that you should probably keep it...
Peter