And now I found a new reason to like it. I was catching up on conversations on Twitter when I came across an offhand mention of Epub3. A friend happened to mention that Sigil had an Epub3 plugin.
She was right (thanks, Moriah!) Sigil has had an Epub3 plugin since at least November 2014.
The plugin takes a valid Epub file and reprocesses it to turn it into an Epub3 file. It doesn't enable you to edit an Epub3 file, just create one, but it's still more useful than a box of rocks.
I haven't had a reason to use it so I can't comment on how well it works. But I do see that the plugin has received numerous updates in the past 8 months.
Update: According to a user report on Twitter, the plugin can take up to a minute to generate an Epub3 ebook. The output needed some tweaking before it would validate.
Have you tried it?
P.S. While we're on the topic of plugins, there are a dozen plugins for Sigil, and they each serve a different purpose. Several of the plugins will help you clean up your Epub ebook by removing and fixing subtle problems (like punctuation, styles, extraneous entries in the OPF). There's also an EpubCheck plugin and (my favorite) a plugin which lets you import Kindle ebooks and edit them.
What can I say, I like doing things backwards.
image by mwilkie