Stems do contain lower levels of nicotine than their lamina. Nicotine is manufactured in the root tissue, is transported through the stems, and is concentrated in the lamina--and also secreted by the trichomes (hair structures) on the upper surface of the lamina. Stems burn well. With considerable effort, I even rolled a cigar entirely of cigar variety stems as filler.
The problem with stems is their mechanical unsuitability for use in cigarettes and cigars, which are wrapped in thin paper or leaf. Big tobacco mostly uses them in cigarettes by flattening them with high-pressure rollers, then shredding. Many cigar rollers frequently keep the lower (tip) portion of stems that are present in froglegged filler leaf. I have sometimes added somewhat thicker segments, buried within the interior of the filler. Stems within a rolled cigar lend it more rigidity, but they're just a mechanical nuisance to deal with. I've tried smoking a pipe bowl filled with nothing but small stem chunks. It smokes well, and burns well, but is troublesome to initially pack, and the little cylinder segments have a tendency to wander into the smoke hole, and plug it.
So, stems are ideal for a preparation like snus, for which the stems are turned to powder. I don't use snus, so I can't comment on their bitterness in snus, but I have never sensed any bitterness in stem from non-bitter leaf.
Bob