At its best, the Showcase is probably the most interesting forum on the site, for me. It's a good way to keep people engaged during the huge gap between releases. I would love it if more people talked about their projects here. It's great to see what people running MyBB are doing.
The problem in the Showcase comes when people who haven't really spent any time or thought on their sites come in and expect glowing praise. There are ways to restructure the showcase that could make it more palatable. A few suggestions:
Web Development and Administration could become "Development and Management" and be a catch-all forum for all web development and community development/administration threads, and "Showcase" threads could be indicated by a prefix. Combining Showcase threads with other threads that are designed to guide users during their community's development would re-enforce a more positive atmosphere.
The rules for the forum could be streamlined to be less verbose while also including a few more useful caveats, such as "common mistakes that could hurt your forum" with things like too many forums, bad domains, etc, and a requirement for a certain level of effort on the admin's part to develop content. Maybe a format like Kickstarter where they actually have to talk about their project, their motivations, their goals ...
If people just want the Showcase to be a place to spam their sites, may as well just disable replies in it entirely and make it essentially a link directory. But this will make it totally worthless IMO.
I realize I'm responsible for some of the "griefing" in the Showcase, so to speak, but it's totally exasperating to see really quality projects constantly getting hammered down the thread list by unsalvageable, worthless new sites.