I’d say from what I’ve seen not enough places use a separation between development and production. Development server lets to update plugins and themes and test if there are any regressions or quirks with your content before it appears online. It’s not a surprise, though, because of the resources needed to maintain two servers.
The only possible overkill is if you ever need to stand up two or more different Omeka sites for different projects. Then, there would be a lot of duplication, since you’d need two different installations on develop, and two on production. There would be a lot of overhead in maintaining that (which is why we’re building Omeka S!). But, if it’s all for just one site, that seems like a reasonable approach.