Is an MVVM architecture always needed?

No, not always.

It depends on the application very much, in particular, on its complexity, functional requirements to UI, need in inversion of control (http://en.wikipedia.org/wiki/Inversion_of_control[^]) and other factors.

In many cases you would prefer you own home-baked approach. Such cases can be either very simple cases, or on rare occasion, extremely complex ones, where you might use the ideas of the well-known architectural patterns or some combination and add something else. However, I would think in majority of cases one of the well-know architectural patterns (http://en.wikipedia.org/wiki/Architectural_pattern[^]) will be the best.

Anyway, it's good to have good understanding of the following architectural patterns:

MVVM — Model View View Model,
http://en.wikipedia.org/wiki/Model_View_ViewModel[^],

MVC — Model-View-Controller,
http://en.wikipedia.org/wiki/Model-view-controller[^]),

MVA — Model-View-Adapter,
http://en.wikipedia.org/wiki/Model–view–adapter[^],

MVP — Model-View-Presenter,
http://en.wikipedia.org/wiki/Model-view-presenter[^].

—SA