It really is a dumb reason to buy a figure - because it "goes with" another one.
I think it's a case-by-case basis, honestly, and, like hmmberto said, depends on your collecting style.
Like,
can you have just an R2 on the shelf without C-3P0? Or
just a Chewie without a Han? Sure- they spend enough time apart that it's warranted, even if it feels a bit strange. Same with Batman and Robin- they're strong enough characters on their own. Someone like, say, Pintel and Ragetti from the Pirates movies? I'd argue you can't have one without the other, just like you wouldn't
just get Laurel and leave out Hardy, or get 2 of the 3 Stooges, as they're all duos/teams meant to go together, and having both/all only strengthens the display, I think. Again, you
could, but you'd be a crazy person, and judgement shall be placed upon you.
With a large, ever-changing team like the X-Men, yeah, there's a bit more wiggle room since there's not really 1 specific way the team
has to be or
should be. You can have a Cyclops without a Jean, or vice versa. You can have
just Wolverine, since he's more or less the face of the team these days. But by that same logic, I don't necessarily think there's anything wrong with deciding in your head what "your" version of the X-Men is, and collecting based on that, whether it's a single existing version, or you mix and match different styles from throughout the years.