I have indeed also met WoW players who reasoned something like this: if X ability exists in the game, it must have a purpose! It can't be the case that it is useless and not worth using; why would the game designers do that? Therefore, the people telling me not to use it must be incorrect.
Of course, this reasoning is incorrect (I leave off the full, general justification of why it's incorrect), but one may legitimately hold the opinion that it points to a failure of game design. After all, shouldn't all abilities given to the player be useful? Shouldn't the aforementioned reasoning work? Why give me a button if I'm never to press it?
Well, there might be several reasons. The more fundamental one is what Monte Cook has called "ivory tower game design": a design wherein the optimal way to play is non-obvious, is possibly obscured or obfuscated by the fact that the purpose of abilities the player gets is not stated outright (only what the abilities do is stated), and further muddied by the presence of options that are not even intended to ever be optimal ("trap" options, or, less disparagingly, "flavor" options).
Ivory tower game design is usually written about derisively, but I am a fan of it. In gentler incarnations, it adds much-needed cognitive challenge to a game (and Blizzard's quest to strip the ivory tower out of WoW entirely has contributed much to the game's sharply decreased attraction for me).
Another reason one might be given a button that one is not expected to use is situational appropriateness. There may be abilities that are useful when e.g. fighting a monster solo, by yourself, but not appropriate when you're teaming up with other people. Other examples abound. Expecting every button to find an application in every situation is unreasonable.
Finally, it may be that whatever the game designers intended, what turned out was something else. A game like WoW is a very complex system. It's difficult to predict the effects of all variables, even when you have huge design teams and vast playtesting resources at your disposal. And so: the only-Steady-Shot rotation, designer intent notwithstanding. Believing that anything that exists in the game must have been deliberately thus designed is akin to another, larger example of teleological reasoning...
There are things that are worthless-- that provide no value. There are also things that are worse than worthless-- things that provide negative value. I have found that people sometimes confuse the latter for the former, which can carry potentially dire consequences.
One simple example of this is in fencing. I once fenced with an opponent who put a bit of an unnecessary twirl on his blade when recovering from each parry. After our bout, one of the spectators pointed out that there wasn't any point to the twirls and that my opponent would improve by simply not doing them anymore. My opponent claimed that, even if the twirls were unnecessary, at worst they were merely an aesthetic preference that was useless but not actually harmful.
However, the observer explained that any unnecessary movement is harmful in fencing, because it spends time and energy that could be put to better use-- even if that use is just recovering a split second faster! [1]
During our bout, I indeed scored at least one touch because my opponent's twirling recovery was slower than a less flashy standard movement. That touch could well be the difference between victory and defeat; in a real sword fight, it could be the difference between life and death.
This isn't, of course, to say that everything unnecessary is damaging. There are many things that we can simply be indifferent towards. If I am about to go and fence a bout, the color of the shirt that I wear under my jacket is of no concern to me-- but if I had spent significant time before the bout debating over what shirt to wear instead of training, it would become a damaging detail rather than a meaningless one.
In other words, the real damage is dealt when something is not only unnecessary, but consumes resources that could instead be used for productive tasks. We see this relatively easily when it comes to matters of money, but when it comes to wastes of time and effort, many fail to make the inductive leap.
[1] Miyamoto Musashi agrees: