It's also a matter of degree. What is the use of regularly needing to say "actually, both parties are corrupt!" if the scale of the corruption (or other political and policy issues) are orders of magnitude different? In practice, it provides plenty of cover for the increasingly radicalized party to continue and expand their actions under the cover of 'unbiased' "both sides are bad" coverage and analysis. Even beyond the potential jading effects on the average not-highly-partisan voter, it obliterates any real chance at nuanced understanding and discussion.
Instead, that sort of both-sideism leads to superficial understanding and inaccurate criticisms. In this thread, NSS, you've said that both parties don't want to do something about corrupting money in politics, but that's plainly incorrect. You haven't really responded to multiple people pointing out to you how that analysis doesn't hold up, but it's the crux of the "both sides" charge here. There are a limited number of things we can do in a CU world, and Democrats would like to do them while Republicans do not. Republican Justices will almost certainly fully uphold CU while Democratic Justices would be likely to weaken it. Yet the problem is the "scumbags" in "both parties." Isn't it pretty clear how that stance lets the party that has actually implemented those policies and opposed reform off the hook?
We see it over and over again. The sitting Republican President called for banning an entire religion; he's built concentration camps for children and he's proposed taking away AIDS funding to pay for it; they clearly stole a Supreme Court seat; he's called Nazis "Very Fine People" while engaging in the very same 'both sides' cover; they oppose any sort of campaign finance reform and celebrate CU and expansions of it to strike down state law; they engage in racial gerrymandering and widespread voter suppression; the GOP is the only major party to actively deny that global warming even exists let alone is caused by man; they have open white supremacists with important seats in the House. Where is the equivalent? What is the use of pointing out that the Democratic party is not perfect in response to the egregious problems within modern conservatism?
That of course doesn't mean there is never room or reason to criticize Democrats, liberals, leftists, or anyone else. There's space and reason to do that now, just check out the infighting our very own Dem thread! But this is about the desire by a lot of moderates to reflexively reduce things to "both sides" at all times. Nuanced discussion requires careful thought, the opposite of that impulse. "Both sides" gives cover to bad faith actors, and it gives legitimacy to bad ideas.