I don't know about anyone else, but I really depend on icons to give me a clue about your post. I experience what feels like cognitive dissonance every time someone, idk, uses a Star Trek icon on a post to a Snarry fic find community or a Merlin icon on their Snarry-related post - especially if they are posting fic. Why would anyone post a Snarry fic using an SPN icon? The only exception is if people are using their default icon. But otherwise I don't understand the meaning behind their choice of icon. Are they being ironic? Playful? Passive-aggressive? Are they having a bad day? What am I supposed to think about their choice of icon?! Because, clearly, they couldn't be just using icons randomly or using a non-fandom specific icon because they don't have unlimted icon space or because they actually like it. Certainly not. Icons are srs bsns and these are srs concerns. And, yes, having this much free time to think of things like this is pretty spectacularly awesome, thanks for asking.
I feel strangely protective of my default icons and I hate seeing anyone else use them. It's the primary reason I actually have my username on the one I've been using for the last several years. Whenever I post I try to use my default icon. The only exception is when I'm posting something fandom-related in a fandom that isn't HP or if I have an icon that is really absolutely perfect for the post. But even then I wibble over using something that isn't my default icon. Because, honestly, I associate all of you with your icons and I assume you do for me as well. There's a reason advertisers repeatedly show their product logo. It inspires product loyalty and increases name recognition. If you aren't using your default icon, it always takes me a moment to figure out who you are. Not that I don't read your posts. Except on very rare occasions (like now), I read every entry on my flist though I may bookmark fic to read later. As we've already established, I have a great deal of free time and an even greater degree of nosiness. So I rarely miss much. But if you aren't posting with your default icon, I actually have to stop for a moment and sound out your user name to figure out who you are.
And, actually, if your user name isn't made up of words that my brain can instantly recognize, my eyes just pass over the letters and I mentally assign you a name that likely isn't your user name at. all. but one I associate with you. It's always interesting when I talk to other fen in person and they'll say something like, "Did you read X's latest post?" and I'm all, "I don't think they're on my flist." And people will give me the strangest looks and say, "Well, they're a huge Snarry person. I've seen your comments to them all the time. I'm pretty sure you have them friended." And I'm like, "Oh, are they the person who likes pie or has poodles or has the icon with the thingie on it?" And then they make a face like they are mentally cataloging escape routes before they suddenly remember an urgent phone call that they need to make immediately far away from me.
And God forbid you ever change your user name. You may as well become an entirely different person. Unless I've met you in person or know you IRL. Then in my mind you are forever "X who goes by some new name I don't remember". I am a slow adapter. (sekrit message to adbaculum - I know you go by another name that makes me think of testicles, but you'll always be adbaculum to me. And I only know how to say your name because you pronounced it for me. Otherwise I always just associated your name with bacteria and gynecological exams. Perhaps the new name was a good choice after all).
Though I will admit that I've toyed with the idea of creating a new identity. I've been in slash fandom for 28 years. I'm only accioslash on the journals. But I have been other names far longer depending on the fandom. And it gives me that same sense of wrongness when I use my HP identity to comment on, say, Merlin fics. I think my user name only makes sense within an HP context and using it outside of HP disturbs my sense of order. I keep thinking I should have an overall name I use for everything. But I am very attached to accioslash. It really is so appropriate that my name translates to a summoning charm for all slash. Because even though I think people associate me with very specific pairings, I'm multi-fannish and there are very few slash pairings I won't try. Plus people actually call me "Accio" at cons and meet-ups and in chat when before in other fandoms they called me by my first name. And somehow being called Accio actually feels more intimate than my real name. Like we're all members of some sekrit club.
I'd ask if anyone else does any of these things, but I think I can guess. Though, as always, please feel free to tell me how batshit I am share your thoughts.