I agree in as much as I prioritize functionality (including responsiveness and lack of bloat) well above a beautiful UI. But I think that most peoples beef with UI/UX is that it often gets implemented at the expense of functionality.

Any UX that decreases functionality is not truly good UX. It is probably just flashy.

Also, personally, I don't entirely agree with your screwdriver metaphor. I have some beautiful old screwdrivers with lovely wood handles that I got as hand-me-downs. I do select them in favor of others for the experience. They are a delight for me to use (but again, most importantly, they are very functional).

Edit: Also, they claim in the title that this is a faster Thunderbird. I would consider that a big plus for functionality.