Okay, that I totally buy. Macromedia's (and Adobe's) tools were pretty abysmal.
And I guess that answers my question in general, too: It's not supposed to be a general-purpose language, and for casual games specifically multi-platform has a lot more cache.
Motion Twin, I believe, use it to power their websites - the javascript, Flash game code, server side multiplayer and web site code are all written in the same language. That's ridiculously powerful for that specific use case.
PHP wasn't an original goal; the first versions of haXe only supported Flash, JS, and NekoVM(A server-oriented VM Nicholas created). The community added the PHP target later since there was demand for using it on cheap web hosts in place of Neko.
Today there's a stronger demand for the C++ target and cross-platform graphics APIs, to get a solution that works on the web and mobile platforms equally well.
And I guess that answers my question in general, too: It's not supposed to be a general-purpose language, and for casual games specifically multi-platform has a lot more cache.
I'd still like to see the PHP use case, though.