The best argument for PHP having it's own web server is to allow novice developers an easy way to code and test their sites. I'll go out on a limb and say I don't think the lack of a built in web server has hurt PHP's adoption in the past.

And a built in web server would only be good for the novice tinkerer. I don't see entrenched PHP shops switching, nor new applications being written specifically for the build in server when Apache w/mods, nginx or LigHTTPD offer better solutions (mature solutions) that are available now.