My experience with bottom leaves.
For my first few years of growing tobacco, I carefully primed bottom leaves at just the right moment, and handled them with care--all the way to finished leaf. The result of that effort was a moderately large quantity of thin, flavorless leaf. That's good for "volado", to improve the burn of a cigar. But it's a lot of work for crappy leaf. And that's the leaf that tends to be (read "always is") dirty, even if you clean it along the way.
One benefit of leaving that leaf on the stalk as it deteriorates into nothingness is that it shades the soil, and minimizes weed growth and soil moisture evaporation. This season, for example, when time comes for the first priming (starting with the second, third or fourth true leaf), I pluck off the trashy bottom leaves, but leave them lying on the soil as mulch.
When stalk harvesting, I place the lopper blades just below the first leaf that I actually want to keep.
As I rifle through my bags of stored, finished leaf, there are bags of "trash" that seem to sit there forever. I'm not inclined to use them for anything. They represent labor and curing space and kiln space and vapor-proof bags that were wasted.
My conclusion is that if you have a tiny crop, and don't mind smoking truly low-grade leaf, then the bottom leaf may be worth the effort, but only if it's not really shabby stuff. Otherwise, just believe the "flyers and trash" designation for the very bottom leaves.
Bob