So let me give you an example of the problem. You have your filter bubbles, but then you visit an independent forum for amateur taxidermy. It has an irrelevant miscellaneous section which is well moderated enough to not be full of spam, but not by someone who really cares about or even particularly understands politics at all.
Someone on that forum posts the following statement. "The concept of white privilege is anti-Semitic because the subset of white people who are doing better than black people are disproportionally Jewish."
The factual component is true, it's not obviously spam, so the moderators leave it there. But what happens when people on the left read that?
It pits members of the same coalition against each other. If you're black you start wondering whether Jewish privilege is a term you should start employing, but you're not likely to be pleased with the response if you do, and that may leave a bad taste in your mouth. If you're Jewish you feel attacked and suddenly nervous about a popular tenet of your party's platform. If you're a non-Jewish white Democrat who has never been exposed to anything like this before, you're primed to receive some outright Nazi propaganda next.
Statements like that need to be encountered for the first time in an environment where the problems with them can be analyzed thoughtfully and without vitriol or recriminations, because otherwise, when they are encountered, they create internal conflicts and push people into the arms of the opposition.
If you ban them from your filter bubble, that is not the context in which they'll be first encountered.