The plastic sleeves on the spring aren't critical to the brakes operation, but they make it much easier to keep centered.
As brakes open an close the spring slides up and down the arm slightly. Friction between the spring and arm doesn't effect braking at all because it always self centers against the rim, but it will affect how they open. A little bit of friction is enough to cause the opposite arm to open more.
Keep the place where the spring oiled and they'll be fine, but it'll attract diet. The plastic sleeves allow sliding without attracting dirt.
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