I'll admit, I run my Mac copies of MC as clients to servers running on Windows, so I usually have those options on OSX set to Nothing. I use the Mac version more and more now, though, as a client.
I have run my Mini with the Media Network server turned on in the past (to re-serve the Windows library, actually, for my own wacky purposes). That's how I know it worked right on 10.9, but I turned that off when I upgraded to Yosemite and hadn't tested it until last night. I suspect my setup is largely the same for most of the developers, at least with their "home systems" they use every night. And, like I said, I've seen other flaky launch-on-login behavior on Yosemite, so it could be that it's just wacky and hard to fix it, and they have other fish to fry.
I'm trying to pay better attention to the Mac sub-board here*. Reporting issues clearly, and keeping on top of it, is the best way to get things done that matter to you.
* To be clear in case you didn't know, I'm a mod here on the forum, but I don't work for JRiver.