ST provides an extra detent and spring as a convenience. You never know when you may slip and suddenly your detent flies out and vanishes into thin air. =)
Wish other companies who sell LPKs follow that lead.. Can't tell you how close I was to breaking something in my fit of rage when one of the detent pins went flying out (specifically the rear take down detent pin) during my lower assembly.. And that being my first build, I was being extremely careful for it not to go flying out.. Luckily I was able to spot it in the carpet after searching for it and all went well afterwards..
Sent from my Commodore 64..