it doesn't aesthetically match a lot of pokemon and then like the ones it does match are usually hilariously easy to catch and you wouldn't wanna waste something like that on it. so i'm like........man.............i'll Pass
It isn't the aesthetics for me. It's the fact that anything is possible to catch without it. I don't even think legendary Pokemon are challenging to battle, with a couple exceptions. Like Mewtwo or Rayquaza. But if my party isn't threatened, I'm probably not using the Masterball.
However, there are those few legendary Pokemon that flee. I usually try and conserve it for them.
I think it looks fine, but I too haven't used it or even been excited to get it since Gen V. I only ever used it to catch legendaries because in my mind it was like, "this is what I have to do." But then I realized I never use legendaries on my team nor have much interest in them and for some reason it didn't even dawn on me to just catch a regular pokemon with it.
The design isn't fantastic - although it's still better than some, like the Heavy, Fast, Level, Lure, Luxury, and Repeat Balls - but I don't use the Master Ball much these days because I don't really need to...and I tend to save it just in case I do need it. Legendaries these days don't usually have an obscenely low catch rate like they used to (at least, I don't think they do...maybe I've been getting lucky?) but with things like roaming Legendaries I just cannot be bothered, so it's faster and easier to just throw the thing at them. I'm only out for dex completion anyway so it doesn't matter what I catch them in when I'll just box them and forget about it. xD
Poke Ball aesthetics aren't really something I consider much when catching Pokemon, so that's never stopped me from using the Master Ball. The only reason I don't use it is so I can guarantee a catch on something like a random shiny Pokemon. That's why I haven't used the Master Ball since I found a shiny Ducklett in White (and the only other time I've seen a shiny in the wild since then was a Vulpix in LGE before I got the Master Ball, so a Premier Ball did the trick).
it's becoming more of a thing where i kind of go out of my way to avoid using the master ball as much as possible. i like having it as a sort of trophy of sorts. considering it's very possible that i can catch legendaries in literally any other kind of pokeball ever (with a bit of luck and some strategy), i don't really feel compelled anymore to use the master ball.