Hmm. I have personal feelings bout what 'little' means to me - to me, personally, it implies a higher level of emotional investment than 'Adult Baby' or even 'Adult Kid'. To me it says 'this person's 'little side' is a big part of their personality and day to day life, rather than a role they adopt or fall into in private times'. Generally, in my mind, it also means the person is older than a baby. As a semi-aside, I've also met numerous littles who don't identify as ageplayers or as regressing in any way, i.e. they seem themselves merely as adults with a childish, nurturing relationship dynamic or an interest in 'kiddy' stuff, no roleplay or 'feeling like a kid' involved - this informs my feelings on what a 'little' is.
As for the argument that 'little' is a blanket term, imprecise and unnecessary on ADISC... well, that's /technically/ true, but I think there's something to be said for people appreciating the ability to use the word that feels best for you, even if it's imprecise. I feel similarly about terms describing sexuality - I could choose to call myself pansexual, polysexual, bisexual or just 'queer'. Of those, I'm always going to choose 'queer' if it's available, despite the fact it tells people very little about my sexual or romantic orientation. It just... feels better, than those other terms. The same way 'little' feels a lot better than 'adult kid'.
I think it's worth noting that I've also met ABs who decidedly do NOT identify with the term 'little', and they wouldn't fit under it as a blanket term - people who see themselves as adults forced to act like a baby or child, humiliated by being put in diapers or treated like a toddler, with no actual regression or that quality of an interest in a childish/nurturing dynamic, or in kid's stuff. Those for whom this is a 'bedroom role' which is less about acting like a child and more like embracing the embarrassment inherent in being a grown adult being stuffed into the role of a child. This, again, is another way in which words' meanings can differ (and still be accurate) when used from person to person. A physically adult person who regresses emotionally/mentally to the age of a toddler is just as much of an 'Adult Baby' as an adult 'forced' to wear enormous poofy diapers and sissy dresses.
Anyway, part of why 'little' and 'queer' feel better for me to use is down to the personal connotations those words have for me, as I mentioned earlier. 'Little' is my identity, rather than 'adult kid' or 'ageplayer'. But I think another part is that for some of us, it's less about pinning down what precisely we are, and more about establishing what we're NOT. Thinking about it, most of the labels I use to describe myself are this way:
queer = not straight,
non-binary = not a girl/not a boy,
little = not a grown-up. If you ask me to pick a single more specific term for any of those things, I'm going to panic a little. Tying down specifically whether I'm feeling elements of Adult Baby-ness, or Adult Kid-ness, or just being a childish adult, or nudging into furry or DL territory... is hard!
I use a pacifier, bottle and diapers - which some would say makes me an adult baby. I enjoy my diapers - which some would say makes me a DL. I feel more interested in coloring books and video games than baby toys mostly - which some would say makes me an adult kid. But I don't know how well any of those fit me, and none of them 'feel right' like little does. Also, if I decided every single one of those applied I'd have a lot more things above my name!
Instead of confining myself to a lot of small boxes that I'm not sure exactly fit, I'd rather sprawl out over the entire spectrum of 'little' and be comfy knowing it tells people the single most important thing about my 'role' - I'm not an adult 100% of the time. And if people need or want more information on exactly what being little means to me personally, they can ask!