Their function is to imitate the radiator air scoops found on the water-cooled bikes that came out around the time the 600 was introduced - in other words, pure marketing. I don't think they contribute anything to cooling. I run my current 600E without them (and two XT350s before that) and I have not had any problems. That said, I do live in the UK, where sustained hot weather is very rare.
Both mine broke, and replacements are hard to come by or very expensive, so I didn't bother replacing. I think the bike looks fine without.
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