The rule that it is a potential contempt to make a serious allegation against the Speaker that reflects on his or her impartiality derives from the longstanding practice and tradition of the House of Commons. The rule serves to protect the reputation of the office of Speaker and the institution of Parliament.
Yes, we should keep it because its tradition. Like slavery, public execution, or corporal punishment once was. And it protects the powerful from the peasants! Yes, that's an excellent reason to do something.
Lets be clear: this is an undemocratic rule whose sole purpose is to stifle criticism of the powerful and deter us from criticising them. Its a sedition law to protect MPs (and Peter Dunne has explicitly used it as such). But like the now-repealed sedition law, if its required, then the protection is undeserved.
As for who they're targeting, it speaks volumes that the new guidance on "use of social media by the Parliamentary Press Gallery" ends with a warning that:
Any public reflections that members of the Parliamentary Press Gallery make on the character or conduct of a member (including the presiding officers) may amount to a contempt.
This is of course the job of the media. And our MPs don't want them to do it properly. And then they wonder why the public holds them in contempt? Once again, they're earning their reputation.