I'm not pushing Sigil or anything, but after you know how--and
can--build an epub from scratch, the knowledge gained is the
only advantage--and that knowledge is invaluable. But it doesn't go away just because you start using an editor designed specifically for epubs.
There's no style points given for manually handling each and every tedious change to the opf/ncx/nav file required by changes to the other files. There's no style points for manually changing the url/anchor in every link throughput your xhtml files just because you've decided to rename an file.
Using an editor like Sigil (or calibre) also means it's impossible to forget to add the mimetype file to a zip archive. It means you don't need to worry about making sure the mimetype's the first member of the archive and is uncompressed. It means you don't have to worry about creating/maintaining zip archives
at all.
Letting software manage the opf and the zip structure frees you from the mundane portions of epub building. It allows you to focus almost entirely on the xhtml/css. It allows you to work more efficiently.
And using Sigil doesn't mean you have to give up CoffeeCup as your XHTML editor. Sigil has allowed epub creators to use external editors for quite some time now.
But you're right, there's no urgency to change. Especially if you actually
enjoy working harder/longer than you need to.
(That's not a personal dig. I realize there's many who adhere to a "
if it ain't rough, it ain't right" type of philosophy.)