Depends on your use-case I suppose. That's why most of these editors are customisable. When writing posts on SO, I've used bold, italic, and strike-through in the same post.

In every day usage, bold/underline/strike may be superfluous, but I would argue that italics are essential. Without at least the ability to emphasise a word in some way, your only option is uppercase.

Programmers love semantic structure, but the vast majority of ordinary people just want to make some letters bigger, italic, purple, etc. It's all about presentation.

Those of us who do care about semantic structure can often do without WYSISYG editors anyway, since we have much cooler alternatives such as Markdown and LaTeX.

Obviously one can do without but it's useful.