The only time we hear a hole in the harmony is when my wife and I sing together and find the same note an octave appart. Then we miss our Alto, she frequents fourths as well as thirds and fifths. We often resolve into what we call "graunchy" chords, the ones that make your hair stand on end and your spine tingle. We really wish we knew exactly what we are doing, then we could use it in other songs but we don't have the requisite musical abilities to explain it, only how to feel it. I think sometimes we use flattened 7th's and possibly 2nd's to get our sound, when you get it though, boy do you know you got it.
JohnB