It seems to make most sense for the already humanoid looking spirits to have a real human form, like Tiennhai or Old Iron though he never did that (I don't think), but though they were giant spirits, they actually looked more like people. The non-human looking spirits, like Raava/Vaatu, AyeAye, Wan Shi Tong, Koh, Mother of Faces,etc. are already anthropromorphic in that they have similar traits to humans, not physically but emotionally. They feel prejudice, anger, determination, hatred, pride,etc. as well as having human like personalities to boot. Like with Raava, the closest thing she has to a canon "human form" is the human host/her avatar, so maybe that's how it is with the non-human looking spirits, they can take a human host and/or enter their body, cause mutations, or kill them. I think that's much more interesting than just every spirit simply being able to turn human at any time they please, because then that wouldn't make them any different than the actual humans. Like any human could be a spirit in disguise and no one would even know it. Not that it's not a bad thing, I'm just saying. I think both good and bad things could come out of it, but imagine if that were the case in Wan's time period. Most people hated and feared the spirits and if one was caught turning back from a human form or vice versa, people would probably flip out and start killing eachother thinking they were a spirit but weren't.