I tend to think the aesthetic of the writing is as important as the content. And sometimes really beautiful aesthetic requires really beautiful, rarely used words. I have no appreciation for most modern art, but that doesn't mean I think it's invalid, or that no one should create it. Or even that it's intended to show huge amounts of pretension. It's just not for me. I guess each author has to consider his or her audience. But more importantly, someone has to take a chance every once in a while to see how the audience responds.
I've also found that sometimes it's fun to use them, and then hang lampshades on them. -- Twenty minutes later, Claire alighted from the stage and left the acrimony behind. Robin smacked himself in the head. Alighted? Really? That's it, no more late nights reading eighteenth century novels. "Claire! Over here!" I've done it once or twice. Also works when I've found myself using polysyllable words but can't really find another word that fits exactly what I want to say or the way it comes out. Kind of a way of letting the reader know I'm making fun of myself for using the word. Not sure how 'professional' it is, but I've enjoyed poking fun at myself that way.
Purpose is part of context too. The purpose for which a word is chosen may well justify its being chosen. It depends on whether we're talking about an uneducated vagabond or an English professor. It depends on whether we're describing a Platonic ideal or a knife-fight. To draw a musical analogy -- I'm a guitarist. If I'm playing basic Chuck-Berry-style r'n'r in A, Fm7b5 is not going to work, no matter how elegant it might be in construction. And if I'm playing Bach, a flat-picked electric is probably not going to work. Use the right tool for the right job. If the main character is, say, an astronomer, referencing "declivity" makes sense. If the MC is a common criminal, that same word will be much more jarring.
Unless of course it's an astronomer who has resorted to crime in order to earn money to build an observatory in his/her back yard.
The worst problem with big words is their misuse, not their use. How often do you hear the word "epicenter" used instead of "center", simply because it sounds more "edjikated". An epicenter is a point directly above a center. It primarily refers to the point on the earth's surface directly above a subterranean event, such as an earthquake. It does NOT mean the same thing as "center", but reporters and others use the bigger word in full bliss of ignorance.
Because one reason to read is to educate yourself. If all authors dumb down their vocab, we'd all be talking/thinking like children. I use 'big' words in my writing and yes I do know the meaning of these words. Is it an ego boost. I don't believe so, because in my case, I actually talk like this. I speak as I type. I'm a lover of words and when reading, if I come across a word I don't understand, I'm more than happy to look it up. But using big words for the sake of it especially if the writer is using it in the wrong context, is stupid. I say use big words if you know the meaning if not use simpler words. Edit; Mervyn Peake who wrote 'Gormenghast' is notorious for making up words in his book- so even if you used a dictionary it would be no help in reading his books. My point is sometimes authors/writers use words that they have no intention of explaining to the reader, which is indeed an ego boost. Self-gratification (a pat on the back) for them and a 'dark alley' for us. To my mind a writer who writes this obscure doesn't deserved to be read. I believe the point of writing is to get something across to the reader.
Response to this thread has been wonderful. Here are few I find particularly useful. I loved this one. "I've also found that sometimes it's fun to use them, and then hang lampshades on them." "Purpose is part of context too. The purpose for which a word is chosen may well justify its being chosen." Use the right tool for the right job." "Because one reason to read is to educate yourself. If all authors dumb down their vocab, we'd all be talking/thinking like children." Unfortunately, here in the US, our education system has been "DUMBED DOWN" so far that reading skills ar at the lowest point ever and writing skills are worse. "The worst problem with big words is their misuse, not their use." Several years ago my daughter asked me to read her Master's Thesis. Later she asked me what I thought about it. I told her "Most of the profs. I had in college would have stopped before the end of the first page." As you can probably tell from what I have written on this site, my writing skill level is low. Fifty years out of college, I still do a better job than my daughter did on her thesis. Now, here is the part that irritates me most. She received an "A" on the thesis. When she received her BA, she graduated Suma Cum Laude. (University of Florida) That was 15 years ago and our grads are getting dumber still. When I received my BS from The University of Montevallo (Alabama) there was no magna cum laude summa cum laude on my diploma but my Mom said, "Lawdy, Lawdy, he made it."