Now Hisao and Misha already know how to write then they meet her, so you'd think that writing things down would be easier, but Shizune doesn't like doing things that way so they have to go with the option that SHE wants.
She seems incapable of even trying to meet them halfway. Maybe I'm remember it wrong, but I can't recall her being extremely happy and grateful for either of them to go through that hassle in order to talk to her. She might be happy that it's more efficient, but she doesn't seem grateful that they'd go through the trouble for her.
That's because Shizune is a type of tsundere - she's not really the type to get all
in front of everyone. She shows her appreciation in different ways - such as the fact that Misha and Hisao end up being her closest friends. The same two people who happened to learn sign language to speak to her. Coincidence? Hardly.
The key thing to remember with Shizune is that for her, sign language isn't just communication - it's
speech. For her, signing with her hands is the same thing as any of us talking with our mouths - the most basic form of interpersonal communcation, and the most intimate.
The reason Shizune has trouble establishing meaningful connections with people is because she can't 'speak' to them. Most folks don't know sign language offhand, after all.
The situation is very similar to, say, a Japanese person moving to America, and being unable to speak to anyone there because nobody knows Japanese.
In theory, this is a simple problem for a deaf person to get around - they may not speak or hear the verbal language, but they still *understand* it it written form. They could pass notes like Hisao did at first, or use text messaging or the Internet and such. But the fact remains that these aren't the same as actually *speaking* to someone - interacting with another person face-to-face, hearing the intonations and subtle emphasis in their words, reading their body language. As most Internet hermits (like me) would attest to, there's a world of difference between the two, and it's that difference that makes it so hard for Shizune to be understood.
Without sign language, Shizune can only communicate with other people indirectly - through the written word, or through Misha (who, as Hisao notes repeatedly, is more or less incapable of delivering Shizune's words on any other setting than 'boisterously cheerful'). She can 'speak' about her work on the student council - but she can't say it with the firey passion she has for the job in a way other people can parse. She can try to empathize with people, but they can't hear the sympathy in her voice. She can't hear the spirit-lifting "Wahaha~!" of a friend's laughter, or the cracking of their voice as they struggle not to cry, or the telltale hesitation and uncertainty when someone is lying to her. It's like having a psychological brick wall between her and other people - she never really feels *close* to anyone when such a fundamental social gap exists between them.
That's why sign language is so important to her - and why Misha and Hisao are important to her as well, even though she might come right out and admit it.
They both took the time and effort to learn how to 'speak' to her, in her own language - her own voice. Sign language might still lack some of the subtleties of the spoken word, but for Shizune it's like having someone climb over the brick wall just to talk to her - and that means a lot.
If she won't, you have to wonder why. Is she embarrassed by the sound of her voice? It would fit the character. That would mean she values her own pride more than the ability to communicate with others.
It's not just her - deaf people the world over share that sentiment.
The problem with speech is that, for a deaf-from-an-early-age person, it's extremely difficult to do it properly. Sure, they can try and learn, and they might even become passable at it - but because they can't hear their own voice, they have no idea if they're doing it *right*. Spoken language depends as much on intonation as it does the words themselves, and when you can't hear such things to understand how they work or how to do it yourself, mastering them becomes next to impossible.
Most deaf-from-early-age people who learn how to speak end up sounding *very* conspicuous, and not necessarily in a good way - it's easy for someone hearing them and not knowing their disability to mistake them for being learning-impaired or otherwise mentally deficient. Obviously, this is *not* something deaf people want, Shizune in particular - she's a smart, driven young woman who wants to be taken seriously, not garner pity. Sign language is a way for people like her to have their own form of inter-personal communication that they can master and wield with confidence, rather than being forced to use a method that unfairly emphasizes their disability.