Is Android designed to not rely on com.android.systemui if I disable it?

I have found that my tablets seem to work just as well with com.android.systemui disabled (if you disregard the fact that navigation bar and recents panel etc disappears) which is acceptable to me. Thus I wonder if Android is actually supposed to be able to ship without com.android.systemui and that it is volountary for a tablet/rom manufacturer to implement com.android.systemui?

I suppose com.android.systemui exists for the purpose of a tablet manufacturer getting the ability to add a module for implementing its own customized branded ui without going to deep down in the source code. Modulized behaviour. Is the module necessary for core android functionality to always work though?

So, to sum it up: Is Android dependent on com.android.systemui package to be implemented (exist) on the platform? Will the Android-platform be unstable without it?


Another question: Is it possible to replace com.android.systemui without signing it with same keys as the rest of the rom?