1. It wastes my time. Sure, I could probably set up my editor to fix this, but I shouldn't have to do so to satisfy someone else's pointless indentation fetish. I've personally never worked on a team where this was an accepted, general guideline. It was always just one guy who wanted this, and did it to every function he touched, adding maintenance headaches for everyone else (until/unless other people finally told him to stop).
2. It messes up diffs. Now instead of one line showing up in the diff, the entire block is often different. And yes, most diff tools have options to hide whitespace differences. Again, though, this adds overhead to everyone who doesn't want this block style. I'd rather not hide whitespace differences, because if someone has added a bunch of inappropriate whitespace (or mangled the block while trying to reformat it to include their new variable), I want to know about it during the code review so I can tell them to fix it then rather than finding it later when I'm editing the file.
3. It doesn't actually help anything. Yes, you get a nice column that shows you all the variable names. What good is that, though? Unless you're putting everything at the top of the function (which has its own set of problems), you're not really getting anything useful from this except maybe prettier code (arguable), because at a glance you still don't really get know all the in-scope variables (not to mention file-scope variables). Moreover, you actually lose something valuable with this style, because now it's harder to determine a variable's type. You're trying to scan left from the name across some indeterminate amount of whitespace to match with the type. This is not typically easy to do, which is why column-oriented data is typically displayed with alternating background colors on each row.