`/usr/local/bin` is in the default $PATH, so making it world writable means anything malicious installed there may end up being called by a system utility that runs as another user than your own.

For your (and Brew's stated goal), installing into ~/bin would be a better result, because it's literally that single users account.

But the Brew developers never want to hear any criticism of their ridiculous bullshit, whether it's security issues or their half-assed dependency resolution.