I have ginormous hands, and play 5'ers. When I play a note on the A string, and then decide I want to add the octave below on the B string, my thumb works just fine for that. If i'm playing the lower note first, or if I plan ahead, I often (but not always) go without the thumb. If you have normal or smaller hands, it may not be that good of an idea, but for those of us who can use the thumb and stay in a good, ergonomic position, anyone who preaches it's "not good technique" is just making up rules that are not based in reality. Bass guitar is an instrument that's not very old, if you wrote "rules" when the thing was invented, we'd all be plucking with our thumbs, and a lot of things done today (like slapping or tapping) would not be included in the "good technique" book.
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