In my last post, I have begun to develop an essentially featural writing system for an as-yet undeveloped sign language. Featural writing systems are extremely rare among natural oral languages, but every system for writing sign languages that I know of is featural in some way. So, why is this?
Let's examine some of the possible alternatives. The simplest way to write sign languages, for a certain value of "simple", would be to use logograms. Just as the logograms used to write, e.g., Mandarin, do not necessarily have any connection whatsoever to the way the words of the language are pronounced, logograms for a signed language need not have any systematic relation to how words are signed. Thus, the fact that the language's primary modality is signing becomes irrelevant, and a signed language can be just as "easy" to write as Chinese is.
However, while logograms would be perfectly good as a native writing system for communication between people who already know a sign language and the logographic writing system that goes with it, they are next to useless for documenting a language that nobody speaks yet, or for teaching a language to a non-native learner. For that, you need some additional system to describe how words are actually produced, whether they are spoken orally or signed manually.
Next, we might consider something like an alphabet or a syllabary. (si5s calls itself a "digibet".) In that case, we need to decide what level of abstraction in the sign language we want to assign to a symbol in the writing system. If we want linearity in the writing system to exactly match linearity in the primary language, as it does with an ideal alphabet, then we need one symbol for every combination of handshape, place, and motion, since those all occur simultaneously. Unfortunately, that would result in thousands of symbols, with most words being one or two symbols long, which is really no different from the logography option. So, we need to go smaller. Perhaps we can divide different aspects of a sign into categories like "consonants" and "vowels", or "onsets", "nucleii", and "codas". If we assign one symbol to each handshape, place, and motion... well, we have a lot of symbols, more than a typical alphabet and probably more than a typical syllabary, but far fewer than a logography. In exchange for that, we either have to pick an arbitrary order for the symbols in one "sign-syllable", or else pack them into syllable blocks like Hangul or relegate some of them to diacritic status, and get something like an abugida. Stokoe notation is in that last category. Syllable blocks seem like a pretty good choice for a native writing system, but that won't work for an easily-typable romanization. For that, we're stuck with the artificially linearized options, which is also the approach taken by systems like ASL-phabet.
For a sign language with an intentionally minimalized cheremic inventory, that level of descriptiveness would be quite sufficient. But, there aren't a whole lot of characters you can type easily on a standard English keyboard (and even fewer if you don't want the result to look like crap and be very confusing- parentheses should not be used for -emic value!) Thus, we need to go down to an even lower level of abstraction, and that means going at least partly featural.
Native sign writing systems have a different pressure on them for featuralism: signing and writing are both visual media, which makes possible a level of iconography unavailable to writing systems for oral languages. In the worst case, this leads to awkward, almost pictographic systems like long-hand SignWriting, which is only one step away from just drawing pictures of people signing. But even a more evolved, schematic, abstract system might as well hang on to featural elements for historical and pedagogical reasons.