Why it doesn't matter what Parler allowed.

Because allowing people to say scary things in public (or mean things, or false things) doesn't inflame anyone.  It attracts people who are already inflamed.

There used to be a joke that went more or less something like this:  There's a couple thousand white supremacist militia members in the US and two thirds of them are undercover FBI agents.

And that's the second reason.  Because we actually want the scary conversations to happen in public. Driving those conversations underground doesn't make them go away. Inflamed people are still inflamed except now they are also legitimately aggrieved. 

We have thousands of years of History that proves that this is true. Short of simply killing everyone of a certain mindset, ideology, religion, or culture, it's impossible to make them go away by forcing them to keep their scary ideas quiet. 

Killing a few simply creates martyrs, not converts. Economic oppression, which was used against the Jews as well as endless other minority groups in this country or that, only makes their determination stronger. If I hate you and you make it harder for me to feed my family or pay my mortgage, I will only hate you more.  Anyone would.  Everyone does.

For a current example, did constantly punishing people for questioning the November election make anyone question it less? Did slapping a "fact check" on a post so that everyone knew that bad thought was happening make anyone question the election less?  If you were in charge of making sure that distrust of the election grew larger and larger, you couldn't possibly have come up with a better plan to turn merely inflamed people into legitimately aggrieved people.

And if anyone is up to something illegal, it's simply better to know about it.

The concepts, the ETHOS, of free speech (and no, the ethos of free speech does not apply only to government) was not come up with on a whim.  It wasn't developed because those old smart guys wanted to say bad things or insult people.  The idea of free speech was developed contrary to our normal desire to force people to be good and it was HARD.

Would you really insist that your enemy be allowed to continually abuse you?  Would you really protect the voice of the Devil himself all in order to ensure that YOU could always speak?  Yes. Because it's important.  Because it's necessary for all ideas, even the extremely bad ones, to exist in the public sphere.  Perhaps even especially the extremely bad ideas, so they can be observed in the sunshine.

Because we know what happens when they're pushed into the shadows.  We know.  Everyone knows.  Everyone so vigorously opposing the ethos of free speech KNOWS.  It's just that for some reason they don't care.