In thinking a bit more about, it's starting to sound to me like a matter of pie. Let me explain.
MySQL isn't really holding back any functionality with Workbench. Not the essential functionality that its users need. It's just holding back some time-saving features from a tool that is otherwise fully functional and enables the same output that its open-source version enables. You pay to get to the end result faster, but you can get the same end result with the open-source version.
Everything that is possible in the Standard Edition (commercial version) can also be done with the OSS Edition. You only trade saving time and ease of use for money....
If you are a MySQL expert who has the knowledge and time to manually perform some steps, MySQL Workbench OSS will be the ideal choice for you. If you want to be able to do more in less time, you would like to have the additional safety-net or you simply want to give something back to the MySQL team - the Standard Edition is your logical choice.
Now think about pie. Last night a neighbor brought over a few samples that his mom had made. I told my wife, "These are OK, but mine taste much better, and I'm not sure why." After all, I (mostly) follow the same recipes that anyone else could, though I do tend to improvise the more comfortable I become with a given recipe.
Perhaps the value I bring is a willingness to make everything from scratch, every time, and the ability to both choose the best recipes and to tweak them as circumstances require.
In other words, I fall into the MySQL Workbench OSS camp when it comes to pie. I'd rather do it myself and have the ability to do so. I could use a time-saving option like Standard Edition (read: pre-made crusts or pie filling), but I'd rather not shortcut the final result simply to save time. I want complete control over the result. Sometimes I'll opt for canned cherries instead of fresh when I make a cherry pie, putting me in "Standard Edition" territory, but usually I want to do everything myself.
I can achieve the same result in either way. It's mostly a matter of personal preference. I'm starting to think that MySQL Workbench's licensing options are much the same: a matter of preference for time over money, or money over time.
In sum, I think I framed the original question wrong. I said it was a question of proprietary versus open source. I think that was wrong or, at least, overly simplistic. It's a matter of time. MySQL is charging those who want to save time in development. That's all.