I think OP meant in a more 'judgement' fashion than the practical gameplay reasons.
I do often join servers and randomly screenshot a whole bunch of BL_ID's before I spawn.
And whenever soemone is an idiot, I screenie his BLID.
But lessee...
Demo mode won't really teach you how to build with its brick limit, or teach you how to fight against real players. Nor to understand community / server etiquette.
The ID argument is not an insult. If I see a 34k preaching to me about how Blockland is even though I've been playing for several years, I will bring their ID up. But there is a limit, I wouldn't say anything it if was a 15 - 20k because they would have been here for a year or two, too.
It has a legitimate usage, and it also has an illegitimate usage (for petty insults or deflection).
Hangon now, I did learn to build well within the demo's brick limit. I used lots of addon brickpacks back then to ensure that I got the best build for my limited brick usage. And I have a theory on how to bypass the brick limit in the demo... I will laugh so hard if it winds up working.
Demos did teach me how to fight against real players, because I would do almost RP-like 'imaginary games' with myself and try to predict what the other would do in any particular situation and how I would counteract that. I also learned how to shoot a target while I am moving, while the target is moving (through bot events), and both. It was pretty darn useful. I also learned events, often to make small event-rigged deathtrap builds. And randomly richard around. Randomly richarding around wound up teaching me a whole lot about the game.
And why no-chatting matters: You're either antisocial, or havent' figured out that you hit the T key to chat. Except in fast paced action servers, in which case it's completely understandable to avoid typekilled.