Animations CAN make scenes a lot better, but also ruin everything if not done right. And I don't just mean not right quality-wise, but also just not the way you'd imagined. Any discrepency between fantasy and actual depiction is taking me out of it. And the less information there is, such as with a few still-images, the more information is filled in by fantasy.
Imagine a scene with an LI that has a very wild character and is depicted as adventurous, rebellious, frivolous, talking about how rough they like sex etc. and then get an animation that looks like they're falling asleep with slow and steady movements without any acceleration, like a constant speed back and forth, which just looks automated and devoid of any passion, while also only featuring 2 or 3 positions due to the amount of work involved in making and rendering animated scenes.
Compare that to the same scenario with still-images instead depicting more poses, featuring more dialogue due to increased length and variation of the act, have maybe some animations that just pan over still-images, add some fitting music to underline the scene and your fantasy will do a much better job at filling in the gaps a half-hearted animation would only ruin.
And animations that are not sex-related are an absolute waste 9/10 times, like seeing a character walk unnaturally for a few seconds. Besides being usually uncanny it's also time the dev could've spent on adding something worthwhile instead.
