It is not essential to get all the skills, but it will just be more beneficial for you if you do. From what I've heard, Multishot isn't a very popular skill and not all archers have it.
However, since it doesn't require skill points to learn the skill, you can learn it any time you want. So if you later feel that you need multishot, you can learn it any time.
As for mastering skills, yeah, I like the idea of concentrating on certain skills, rather than putting 1 skill point into each skill you have which doesn't really help.
Learning doesn't require skill points, but empowering (mastering) them does, just in case you didn't know. o:
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