The argument can be made that an outdoor venue is never an ideal listening environment anyway, so other uncontrollable factors are going to have a bigger impact on the sound than a tarp covering the speaker.
Outdoors there are (in theory) fewer nearby reflective surfaces and folks that have never mixed outdoors will be very surprised at how much interior reflections have affected their listening experiences. The reverse is also true - when you're used to hearing mostly the PA with few reflections moving indoors can prove interesting.
Tarps just add an out of sync percussive element to the stew. Yum.
We had subs behind some pipe & drape last week, the tracks for the fashion show would make the drape billow out about almost 2 ft. At least it didn't audibly flap around...