Plan9 doesn't use plain text commands really. Look at how various services work and you'll find there's usually a C library wrapping the underlying pseudo-file. Plan9 was IMHO to a large extent a failed experiment; the filesystem API wasn't extended enough to be able to express enough operations (e.g. no transactions).

The other problem with Plan9 style designs it the concept of filesystem servers. This kills off a lot more expressiveness still e.g. you can't move device files around to organize them better because they aren't really files and the layout is fixed by the server. A dbus that exported stuff via the FS would have the same problem. It'd be full of "files" that you can't rename, move, delete, add xattrs to, read, etc. Most of the operations you can do on files just wouldn't work so what's the point.