You can trust the minifier as much as you want, but it can still have bugs in it. Debian trusts Firefox (Iceweasel), but that doesn't mean it's not full if security holes!
What would be the point of exploiting the minifier? The purpose is to get the code to the Debian users, how does exploiting the minifier help with that? If you can put an exploit is the program's source, you just want it to be minified as-is so that it'll run on the end user machines.
It has the answer to your question. Namely, you can write bugs that are exploitable that aren't present in the original source, that only appear in the minified output. Which means that a) it is a whole lot harder for someone to find (especially if it's something that is "obviously" correct), and b) it's plausibly deniable.
No, you want the minified source to do something different but predictable so that nobody reading the source could spot the backdoor without knowledge of the bug. Here's an example: