It'd be extremely helpful if we could tell the IDE to automatically break into the debugger when the value of an item in the Watch tab changes.

why not?
Just add a global breakpoint and set condition.

For example if a variable $a is 'blah blah blah' and you want to see when it changes, the condition would be
$a !== 'blah blah blah'

That's a usable work-around for now, but it's not the same and not nearly as good. For one, it requires that I enter the value of the variable, which may not be fun - perhaps a big array or even some binary data. Also, as soon as you hit the next break, you must update the breakpoint's condition; you can't just do a quick observation of the variable and hit Run again. If you're trying, for example, to identify the particular loop iteration that's sending a variable's value into never-never-land, this would get very tedious very quickly.

However, I can see how such a flag in the Watch tab would be implemented under the hood using conditional breakpoints that are automatically set and updated... perhaps a path to quick implementation?

I'll think about it, thanks.

I agree. This is very important feature to add. Thanks.