I can see an argument for wrapping it in a macro so you can turn off nulling in debug builds (ASan might even have hooks so you can automate this, I know Valgrind does). But use-after-free is worse than just double-frees, and if you read a dangling pointer in production there's no real way to catch it AFAIK. Last I heard (admittedly been a few years since I checked), you're not supposed to deploy ASan builds because they actually increase the attack surface.
So, your program's memory is full of these dangling pointers, and at some point you will have a bug you didn't catch and use one. And you can't even write an assertion to check that it's valid. What do you propose?
And again to clarify, I'm not trying to advocate for hiding bugs. I want to catch them early (e.g. with assertions), but I also want to avoid reading garbage at runtime at all costs, because that's how programs get pwn'd.