even the most basic struggle mechanic is a nuisance
That's what we disagree on. One of the players characters' main objectives in these games is usually to avoid getting raped - trying to break free from an enemy that's got hold of them is a logical course of action. Without struggle mechanics, they will always fail to do so - which can work in some circumstances. But with them, it keeps that part interactive, retaining agency on the player's part - to either try to break free, or decide that the character is going to give up and let the enemy have their way. Failing to escape or deciding not to try, either way makes the scene more enjoyable for me, and succeeding in breaking free makes it more enjoyable once I inevitably do fail.
Bad struggle mechanics are often redundant at best, yeah. Even the "better" implementations of button-mashing are a foregone conclusion - you're either able to mash fast enough, or whatever determines the difficulty at that time means you simply can't - either way you're ruining your keyboard.
Though I'd still say in most cases it's better than the alternatives - It succeeds in avoiding the player being forced to just sit and watch an entire animation each time one is triggered, without completely undermining the existence of those scenes, "breaking continuity" by allowing the player to just skip them entirely.
Good struggle mechanics on the other hand are an extension of gameplay in and of themselves. What makes for good struggle mechanics is some level of elasticity - you always have the opportunity to succeed in breaking free, but you have equal opportunity to fuck up (the balance can be played with depending on the enemy, PC's status etc.) throughout the course of an animation. This means more depth than just mashing left/right in sequence. It might not be
fun in and of its self, but it engages the player in something they would otherwise just be spectating, affording them agency in the game's most important aspect.