Are you talking about the spring that pushes down on the two hop arms?
The one to the right of this picture:
I personally like to take mine out becuase I like how the hopup acts without it. Your gun will still work without it. What that spring does is add more pressure on the hop arms so you can have a more constant and even hop setting.
A little lighter, it makes it easier to dial in the hopup in my opinion. Plus there's no added stress of making sure that thing stays in the right place when sliding it back down into the gun. Even without that spring in, you still have that O-ring holding down the hop arms. If you're running a crazy hot weapon like 550+ then you might want to think about finding a replacement. Then again, i'm shooting 550+ and I still don't need mine. Every gun is different I suppose.
I allowed my hop up to sit for over two weeks hoping that the hop up levers would start to press down on the adjustment screws. Currently there is alot of slack and I doubt the hop up would be engaged on a bb. The only thing pushing the levers somewhat down is the larger of the black rubber bands supplied with the hop up.
Does anyone use a spring from something fairly easy to find so I cannibalize it. I have lost two springs now. I will be more cautious from now on but I have a feeling this wont be the last time it happens.