I recently purposely left mine off for fear of blow-off, and additionally only added about 1/3 of the water to the 'Max Fill' line. No problems on either account - missing lid was no problem and the water would have taken a week or more to evaporate at in the mid-60s. I think the lid will keep fruit flies and other bugs from crawling in but otherwise is not necessary (that's the wonderful thing about S-locks - the lids don't affect the functionality of the airlock itself).
On a separate occasion, I used the lid with 1/3 water fermenting a saison at around 90 degrees. I DID run into the evaporation issue there, but the solution was to just add another tsp of water - problem solved.
If you're worried, then you can certainly put a small patch of aluminum foil over the top to loosely seal it.