If we're talking about someone who has received a binary copy of software, then isn't this obvious?
The MIT license permits the distributor to close the source of what they've redistributed, in original or modified form. Potentially depriving the end user of the freedom to view/modify/distribute the source.
Permissive licenses prioritize rights of the software redistributors at the expense of the end users.
The MIT license permits some other developer to fork the source and close it off, but as an end user of this particular software that is under MIT (meaning that source is available, and I can take it and modify it if I need to), how does that affect me?
tmux is MIT-licensed, right? The MIT license is very similar to the (3-clause) BSD license which makes it upward-compatible with the GPL (you can incorporate MIT- or BSD-licensed code with GPL-licensed code).
Edit: and to your point of a distributor withholding the source: yeah, so? If there ever came a point where the current maintainer closed its source (unlikely), somebody with a copy of it can step in with a fork. Or the project can die a deserved death for closing its source. At this point the benefits of open source are pretty much obvious to anyone with a brain, and closing the source of an open-source project is practically suicide.