I think someone touched on this but Sprint Runs are very profitable for a knife company. Designing a new knife is very expensive when you look at design, prototyping, tooling, etc. With Sprints, the very expensive tooling essentially stays the same but they can switch our colors, steels, scales etc. And, they get to charge are higher price because it's a "Sprint Run". The PM2 is a great example. The standard PM2 is great seller but getting a bit dated and subject to increased competition. Spyderco switches up the steel and scale material/color and they've created an entirely new secondary, "thirdary, fourthary" market. I'm guessing the majority of the Sprint Run buyers already bought the standard version previously. A customer who would normally just buy the one standard knife is now buying 1, 2 or 3 more. That's pure bonus for the knife company and it's comes at a substantially lower cost.
Personally, I'be never really gotten into Sprint Runs. I think the only one I have it a Hap40 Stretch and I bought specifically for the updated steel. Not you're typical Sprint Run customer.
That being said, I have nothing against all the Sprint Runs. It's a huge profit maker for the knife companies and it's giving knife collectors some great and unique options on standard designs.