One assumption falsely made, albeit an innocent assumption, is that a plastic lid actually seals out air. As many can certainly attest, even with a lid snapped tightly down, rubber grommet in place an an airlock. One may see little to no activity in the airlock because lids even with their soft sealing ring inserts don't actually seal well. I had this happen several times with different fermentations. I was using a low foaming yeast in a room with temps in the mid to upper 60's. While my hydrometer reading told me fermentation was going on, there was no activity in the airlock. On a whim I wet down the seal in the lide and reclosed the container. Viola! activity in the airlock - for a few hours until the moisture helping the seal went away.
What that means in simple terms is that a tightly snapped on airlock is not a guarantee that you have sealed out much of anything. Might as well save yourself the trouble of snapping om and taking off that plastic lid. As meadmaker1 says, the pain of taking off and putting on that lid acts more as a discouragement for doing a proper punch down than it does as a seal to keep out air. So loose lid with a towel, or towel only.
The only thing I would add different is that I just us a towel but I tie a cord around to help keep out fruit flies and other critters. We all know that fruit flies can't hold their liquor so once drunk on your new wine must they can't find their way back out, then they die and contaminate a wine must. So I use a towel to help them fruit flies practice abstinence.