I mean you don't have to, but why not? It'd be cool to have like 5 MAs to make combo techniques with. xD
Cause the thing is, you need swordsmanship for armed combat. Rokushiki for unarmed combat. Life Return for a lot of supplementary techniques. And then you need 1-2 more to add all sorts of elements with Dials and so on.
And here we are not being able to use more than 7 techniques under one MA in an island, let alone having like 7 MA and like millions of techs. Lmao.
I would go with what Ichi says, having too many skills might lose your chars flavor, so there has to be a focus on which one will be your primary.
Should you choose to be a sword wielder, you can have elemental supports to make it more deadly like what Ziosa says, toxia or electricity.
Mine is another case, I am primarily a blacksmith with an advanced methodology that also allows me to dig deeper into science and thus learn Navigation/Weather as well, and invent new things (Science). And by being a smith, I need to have a separate and distinct MA that uses Hammers in combat.