I've been using delta for a long while, but this is the first time I've heard of difftastic. Why wouldn't you want the context all the time?
I feel the visual eye-candy provided by delta far more appealing than what difft does.
Also, delta is much more configurable and has nice binding for moving through hunks.
That's probably the only reason, otherwise, you are correct. I would want the context always.
I use delta as my daily driver but sometimes when I want the contextual info, switching to `env GIT_EXTERNAL_DIFF=difft git log -p --ext-diff` gives a better picture.
[0]: https://github.com/Wilfred/difftastic
[1]: https://github.com/dandavison/delta