Why would posturing be silly? As you say the main chamber(s) of most parliaments is rather symbolic in nature. But it’s important as such: it represents to the public a lot of what their representatives do.

There are usually tons of formal rules for etiquette, dress code etc which are also in a way posturing, or in another way, showing respect for the institution and processes and showing that they take the job seriously. That something is only superficial does in no way mean it’s useless!

Showing on opposition speaker the same respect that a 10 year old shows their teacher in school seems like a pretty low bar.

The point of forcing everyone to pretend they are listening is the same as the point of forcing everyone to wear a tie: to not make it look like a condo board meeting.

What if the opposition speaker decides to speak for 10+ hours (assuming the rules/etiquette of the house allow doing so) because he wants to prevent the government from passing some legislation he strongly disagrees with? Should the entire house be forced to sit attentively listen to him reading TV scripts or whatever? Even in less bizarre cases the opposition speaker might purposeful asking irrelevant questions/talking about irrelevant points to appeal to his supporters or otherwise being unconstructive.

I'd say most of the formal etiquette and dress code rules are needless distractions which historically were designed either to exclude lower class people from government or as a form of a public ritual which would lend additional weight and gravity to decisions made the parliament (because the majority of the population were not educated enough/didn't have enough time/etc. to take them seriously just based on their content).

> What if the opposition speaker decides to speak for 10+ hours (assuming the rules/etiquette of the house allow doing so)

I think the answer to this is pretty obvious: if you are touching tiny issues with chamber procedure and etiquette but have this problem, perhaps start with this and return to dress code and cell phones once the chamber is functioning at all. Again: the idea with “no phones” completely rests on the idea that there is a minimum of dead time in the chamber. If that can’t be arranged then that’s the issue to address.

> a form of a public ritual which would lend additional weight and gravity to decisions made the parliament

It’s this. And it’s the same reason people wear suits in a courtroom. It’s because showing that something is serious has a point. Being superficial doesn’t mean pointless.

> completely rests on the idea that there is a minimum of dead time in the chamber

I don't think we can really compare public sessions in a parliaments to regular business meetings. I'd say their main point is to broadcast a summary of decisions (which are largely taken behind close door and the actual voting is just a formality) to the public in a somewhat transparent and formalized way. I would expect a competent MP to generally know what another MP would say on a given topic before he even stands up to speak so unless he's directly participating in the discussion it shouldn't really matter whether he's using a phone at that moment or not.

> It’s because showing that something is serious has a point. Being superficial doesn’t mean pointless

While I'm fundamentally not against maintaining some kind of minimal dress code requirements, I think in modern, advanced societies that mainly serves an utilitarian purpose (by minimizing unnecessary distractions). The assumption that somehow the more distinguishing dress code or complex etiquette rules a specific public procedure has the more seriously it's taken by the society sounds bizarre to me. It obviously made sense in the past when status symbols like various non standard suits, gowns, robes and wigs had a point in the past because they signified that the person wearing them was somehow qualified and/or entitled to perform a jobs he's doing. But I don't think many people now would believe that a sentence passed by a judge who wears a wig is somehow more 'serious' compared to if he/she was instead wearing just a gown or simply a suit.

Adhering to social norms in civic activities signal buy-in, or not.

Sure, wear dirty hiking boots to the ballet.

Snort and fart thru the sermon.

Talk loudly in the phone riding crowded publuc transit.

Every day is halloween and you need to unambiguously alert all other participants the esteem you hold for them, or not. So choose your costume wisely.

Well I don't see how any of your examples are equivalent of wearing something but a formal suit (as long as you don't have too much dirt on your shoes). I mean if somebody like https://en.wikipedia.org/wiki/Lord_Buckethead happened to be elected to parliament (as unlikely as it would be) I think he should have the right to wear his costume during sessions regardless of the opinion of other MPs because wearing that costume is a significant part of his political identity.

And how should the MPs antagonized by Buckethead respond?

AFAIK there are no formal rules on dress code in the Netherlands parliament. There was (or is?) a member of parliament who attended in jeans and a T-shirt (because that's what the people he represents wear).

Dress codes are usually set to formal in order to mark that the representatives are not representing themselves (by not allowing personal style - even one that mimics their constituents). That is: the anonymity of uniforms instead of allowing personal style underlines how the person is merely a representative and not there in their own capacity. The most extreme version of this is of course robes and wigs in some courts. Like everything else, this is about to change (as it is looking increasingly weird) but that doesn't mean the underlying idea is crazy.