For instance, a spice rack. Sure it's cheaper, but in the long run, wouldn't it be smarter to buy a large bag, which has equal storage capacity, and is more versatile? If bags are to be specialized for a certain purpose, they should have more slots to compensate. Especially since empty bags can't be sold, purchasing a special bag is a short-term benefit only, no?
The existing big bags really give tons of versatility. For instance, I know my green bags contain my tools, my blue bags contain food and ingredients, and my browns contain alchemy-related things. Specialty bags are mainly only special because they LOOK different, but if we could do this if we were allowed to custom-color our bags (similar to what is being suggested in the seamstress area of things).
The tl;dr is that speciality bags should really do more than look different and organize items a little better, either by giving much more space for that particular type of item or by allowing more stacking, or something else entirely.