Anime is not film. It usually has more detail than western animation but people and movable objects are still large flat areas of colour with black lines surrounding them. The animation tuning is till recommended for anime.
Why it is recommended? By who? For what type of encode?
It is true that anime is not film and tune film wasn't made for anime to begin with. And it applies some stuff that I consider bad for anime. But it produces better results than other presets. You talking about flat areas but this is exactly one of the reasons why tune animation is bad. These areas may look flat to you but they are gradients very often. animation tuning will make them flat indeed but this is not what you need for high quality encode. This is more true for 8bit encoding than 10bit of course. I'm not saying that film preset is "best" or something. It may introduce ringing etc. For "best" results when you want to preserve as much as possible, so called visually lossless encode, none of the presets actually fits for anime encoding and custom commandline needed.
Personally I only say tune animation is good for 3d stuff because x264 devs was saying so. I don't encode such stuff myself so not sure. Other than that tune animation is good only for very lossy encodes when you don't mind bluriness, banding etc. and/or if your crf too high (anything above crf18 for anime is not good quality and tune animation may hide artifacts)
What i'm saying doesn't need citations. Anyone who have experience in anime encoding will agree with me, more or less. Well they may recommend tune grain instead if you want preset and not custom commandline. But this is as much suboptimal as tune film (still better than animation)
If your goal is 70mb per episode encodes for use on your android smartphone - this is when you go tune animation. Is it necessary to use tune animation
? Answering "yes" to such general question is pure misleading. You don't know what goals user have in mind. Not to mention that what kind of processing user does before encoding also matters, does "flat" areas really flat or not, etc. For average unprocessed anime source with high quality encode in mind, tune film is what good to recommend for newbies. Or make them learn and chose optimal commandline that will result what they expect to see. This is what I recommended. If you don't agree, you probably also want to learn about anime encoding or average user expectations.