Shift spring

[sprocket74]

I was racing on Saturday and came across a stalled rider while I was climbing a hill that required a bit of momentum so I stomped down on the shifter to make sure I got by him, anyway the bike wouldn't shift up or down and was stuck in 3rd gear (thankfully it was 3rd.) That night I pulled off the clutch case and noticed that the spring on the shift mechanism had broke and fell off which leads me to a few questions.

Has anyone else had this issue?
What is the purpose of the two piece shift mechanism with a spring (Jap bikes are one piece?)