A friend was offering me some patches for my jacket, and as I sifted through them I realized that I didn't want the one that said "acab". Do I believe the policing system is fucked up, rooted in classism and racism, and perpetuates structures of marginalization? Yes. But something about the phrase "all cops are bastards" doesn't feel like it fits with the kind of leftism I want to engage with.
One thing I don't like about "all cops are bastards" is the focus on the individual more than the system. People choose to become cops for all sorts of reasons, and often those reasons are rooted in the systems that they're socialized in. They all contribute to the perpetuation of the police system, but I wouldn't be surprised if some leftist education would change some of their minds about policing. Police as individuals aren't irredeemable or pure evil, and it's important to me that my leftism focuses on dismantling systems instead of directing anger towards individuals.
Another big factor is that my values are very centered around kindness and curiosity. Anger and shame are emotions that are helpful for bringing my attention towards important things and less helpful for sustainably motivating me towards change. Phrases like "all cops are bastards", "queers hate techies", or "eat the rich" feel more grounded in anger and shaming. I know there are others who are easily paralyzed by anger and shame, and I want to be able to reach those people with my leftism.
To take a related example, when leftists say "abolish the police", some interpret this as a desire to take society as it is today and immediately remove the policing system. This is what some leftists mean, but I think it's important to add some nuance -- especially for those who are new to learning about these ideas -- and discuss what would replace the police system's supposed role of enforcing rules and keeping us safe. To abolish the police, we need ways for people to call for help when in danger, ways to incentivize people to not hurt each other, and ways to hold people accountable to harms they've caused. I'm not saying the police do a great job of that right now, but to say they don't help at all with those things feels like a dramatic simplification. We need to move towards a society that can support each other with less reliance on people in positions of law-enforcement, but I think we're too individualist and isolated from each other to do a good job of that right now.
I wouldn't say that no-one should be an acab leftist. There are a lot of different roles to play in activism, and short phrases that are punchy and sometimes inflammatory have been very effective at catching people's attention and sometimes calling them to action. These phrases can also be a great way to hype each other up and build solidarity with each other (sometimes at the expense of unnecessary exclusion of others). But there's a lot of work I want to do in kind education and curious exploration, and "acab" doesn't feel like it fits with those goals. Maybe I'll make a patch that says "the policing system is rooted in systems of oppression, and we should support the oppressed and also develop robust systems to keep ourselves safe that don't rely on police" ...but that's definitely less catchy.