I have no opinion on its value or investment potential, but I certainly consider it collectable. It's a rare knife of a rare type, of interest to collectors of edged weapons or unusual weapons or unusual knives -- you could even build an interesting collection of nothing but nonmetallic knives.
Using it for a letter-opener or similar mundane uses would dull it quickly, and sharpening it every time it gets dull would wear it down to a sliver ... of course it would still serve as a letter-opener even if dull, but if you'd rather keep it in original condition as a collector's item no one is going to laugh at you -- I certainly wouldn't use it to open letters. If you want to use a nonmetallic knife to open letters a $5 zytel knife will do fine -- the Emerson deserves to be preserved.
-Cougar Allen :{)