I think they cover those hole to keep the dust out. they also ussually keep one hole open. Most carmakers used to have rubber plugs you could take out and put back in, but some are now using the clear plastic for some reason, but it is common. the hatch is a lot more watertight than a door so leaving the tape should not be a problem.
There was a thread about this somewhere and I can't find it now. I believe that these were determined NOT to be drainage hles but rather holes for blowing insulation/sound deadening material into the gate. removal is not recommended as water can get into the gateand damage the stuff that was sprayed into the gate plus some can come out as dust. I just wish I could find the thread now.
I remember the thread referenced earlier. Kinda. The drainage holes were only functional when the tailgate piece was dipped into something during the production process. Probably some kind of anti-rust stuff or primer or something. The tape is there on purpose and should be left there.
I take my car to the jet wash and blast the bejesus out of it with the water jet (from a decent distance ). I have yet to see so much as a spot of water on the wrong side of that clear tape.
i beleive you are right about the primer dipping . its epd or something . i know most manufactures are dipping instead of spraying due to the epa laws . this way they can use a better primer but get around the spray booth and epa laws.