I have struggled with that very question many times. In general, if a knife has been discontinued, I try to find a backup (as you did) before using the knife.
Realistically though, most standard modern production knives will not become collectible in our lifetimes. When my parents were kids, they bought comics to read, and read them they did. Nobody back then bought a comic, bagged it and stored it in an archival container. It was the rare comic that survived over the years in mint condition - that's why they are worth so much.
The same can be said for knives. In the "old days", knives were tools to be used, not bought and stuck in a closet or display case. That's why some knives are worth so much - unmolested examples are all but non-existent. Today, with knives made in the tens of thousands, particularly by large makers like Spyderco, mint condition discontinued standard models are not very rare.
Bottom line, if you like the knife, like carrying it, then by all means use it. If you really need a backup, post in the "Wanted to Buy" section of the Forum, and I'm sure that there is someone out there who'll help you out.
Just my opinion, FWIW.
Edited to add: Knifecenter has them in stock for $50:
http://store.knifecenter.com/pgi-ProductSpec?SP52SBK I wish it were that easy to backup some of my knives!
. Welcome to the forums!