Leaflady,
Yes, the tube is for keeping the reservoir filled. "Self-watering" is more about the plant's ability to draw water from the reservoir as they want/need to.
Some rain water will be caught thru the fill tube, and some will be caught thru the plant hole. If you prefer only rainwater, which, I agree, is God's plant tonic, you should probably capture it in another vessel or rainbarrel, and fill the eBuckets directly from those. Remember, once water comes out the overflow hole, your reservoir is at its maximum capacity.
