It does have the advantage of both immediacy and continuation.

You can dump statements through a function and see it's progression when run without having to interact with it.

Throw in something like FirePHP (which allows dumping pretty much anything out as a viewable/collapsible trace, other languages have similiar) and the use case for a lot of using a full blown debugger is removed (it's still incredibly powerful when needed).

So I use both.

Codebug + Xdebug, setting a breakpoint and getting a full REPL at that execution point is far more useful to me when I'm in PHP land :)