I asked myself (still do) the same question. Viewpoint development (Kitalpha) versus plugins/P4C scripts.
I managed to do quite a lot using only plugins (with menu contributions, handlers and new classes that didn’t extend anything) but eventually hit a dead end when I wanted to add a new element to the Capella model (see my topic here).
The documentation is lackluster for Kitalpha (and often confusing) so there is a lot of trial-and-error involved. Unfortunately, however, I’d say it is unavoidable if you’d like to change the model in any deeper way as I found out recently.