They're easier for non-programmers (I work with a lot of designers) to create and use. In fact, most of the plugins use gems and then add their own config and assets.
If these non-programmers are using Git and adding Gemfiles and such, is adding a dozen line gemspec to the repository that big a deal? And if it is, couldn't you fill out most of the gemspec automatically using a command line script?
None of that would be particularly difficult. And nothing here is stopping that from happening, or replacing it. It's just another, fairly simple way, to organize and share code.