Having STS onboard is additional money for the partner = partners will have more % off the price when ordering certain products.
When STS was free and online - any partner could get STS. No matter was the exam passed on the first attempt or on 8th :) More than that - it was not the person who logged to PartnerNet actually passing the test.
Result - lots of STS for partner, good prices, BUT NO actual specialists! When it comes to Symantec products deployment - nobody was able to do it. There were no real STS.
Passing STS on Prometric helps to ensure that Partner will invest in sepcialist education and will have ability to support customers.