If you had to use a voice synthesiser like physicist Stephen Hawking, I imagine you'd leave a few things unsaid in the course of a day, just because it takes longer to type them than would seem worthwhile. You'd probably want a dedicated button for each of a few very common phrases like "hello", "thankyou", "excuse me" and "sorry I ran over your foot with my wheelchair" but beyond that, I can't imagine how frustrating it must be to use such a device for all daily communication.
Mokalus of Borg
PS - On the plus side, if you're giving a speech, you could type it all out in advance then take a nap.
PPS - Assuming there are no typos, questions or technical problems.