I understand exactly. My collection(s) consist of several large lines: (Star Wars, LOTR, Star Trek, TDK, Terminator(s) and Space & Aviation as well as military themed figures). But I also have a large number of "one-offs" or small sets of figures that I picked up simply because I liked the films and they looked cool. Some, like "The Matrix" are really just a half-dozen figures. But most are single representations from films that I enjoyed, such as "The Rocketeer", "Oblivion", "Starship Troopers", "Blade Runner", "Mad Max", etc.
On numerous occasions, I've thought about how best to trim down the collection and it really comes down to whether I want to keep those large lines and sell off the smaller/one-offs or vice-versa. That's been my dilemma.
Exactly this. I often ask myself just how much I like a character, versus how much I like the idea of having the figure for completionism purposes. A part of this hobby's appeal is that we buy relatively high-end stand-ins for fictional characters and properties. And that makes things tricky. You may derive genuine personal enjoyment from a certain IP, but still be tempted to get figures from something like a cinema classic purely because of how important these characters are to the medium and pop culture. Then there's the "enjoyment scale", and having to choose between something that sits at a 10, and some 7s. And that just creates problems if you're trying to juggle it all. One can easily disregard something he has no interest in, but it's those things we (really) like but not exactly love that create problems.
I myself have expressed that I want to build an X-Men display with the MCU heading there. I'm not beholden to any particular iteration, so I'll just pick and choose. A Cyclops, a Jean, a Magneto, a Fantomex and so on. The problem is, I also have the Fantastic Four and the rest of Cosmic Marvel. And Star Wars, which I've already tried to restrain by limiting myself to the Jedi & Sith. But then there's Batman and maybe one day the DCEU will reach the Green Lantern corner and so on. On top of that, I'm drawn to certain classics and characters which, as you've said, act as one-offs. I've not watched any Trek beyond TOS, but still feel the need to get a Kirk and Spock. I bought a Dam Vito to put next to my BCS Tommy Shelby, even if I downright loathe the show at this point. I'll never buy a Predator because I was never a fan, but I'd still pull the trigger for a new T2 T-800 because he's that much of a classic. To keep it somewhat LOTR-related too, I've had the books sitting in my shelf for years. By the time the PO is put up, it's likely that I'll have read them and liked them a whole lot. Even now, purely due to the movies, Aragorn and Sauron tempt me. What will happen then? Will I drop the half a grand on Aragorn, being such an important fictional character, or will I buy another Batsuit because I'd already set up the Batshelf?
It's these things that really complicate the hobby, and sometimes I wish I could pick one or two things and stick with them. The problem is that none of it ever goes deep enough for me to commit. If I was getting High-End EU Star Wars, or Comic-Based 616/1610 Marvel figures, I'd be more likely to make a commitment. As it stands, I'm always cherrypicking since we're dealing with adaptations, and with this scale being what it is complete set-ups are never guaranteed. So this hodgepodge way of collecting ultimately makes more "sense", even if it's a much bigger hassle.