As far as a business case goes, there's no real "vendor lock in" with a plaintext editor like this. You can always switch to something else and your files will be fine/the same. You're investing (potentially non trivial) time to learn using it, but it didn't take me long to learn ST2 to a depth that made it really handy.
I will say I'd buy a lot more into that argument for people who develop plugins.
That said I can respect the "open source" only mindset. I'm just not sure I'm that concerned about it in this particular case.
For example there was a time when I switched a Linux distribution each month, then at some point I realized that it is getting me nowhere: I didn't know either of them in-depth or how to deal with their specific problems. So then I made a choice: I'll use only Debian, and actually learn about how to deal with problems when I encounter them, instead of jumping ship to another distro.
Its the same with text editors, I eventually settled on Vim (for a long time without any plugins), then started using plugins as well.
Sure every now and then I check what new alternatives are out there (for example there are some interesting ideas in http://leoeditor.com/ but not really comparable to Vim), but closed-source tools are never on my list. I only ever heard about Sublime Text here on HN, and even then for a long time I thought its a Mac only tool.
But with closed-source tools I just wouldn't have been motivated to stick with any of them: if there were problems with it then I wouldn't know how to work them around, or implement missing functionality, not to mention I wouldn't trust them to begin with etc.
I should say that I can't really settle on VIM right now because I'm doing full time .net work at this point. I still use it reasonably often when I'm mucking about in C at home, and I used it as my primary editor for at least five years prior to that, but my use-case was me trying to get a decent haskell environment up and running on a fresh linux VM as quickly as I could. ST2 worked really well for that.
If you work on closed-source software then you probably don't care about this / doesn't matter there, but as someone who spends most of his time working on open-source software I definitely want my editor to be open-source.
I'm not going to be forking an open source text editor if development stagnates.
With closed source you don't have much choice except running an outdated chroot with an old distribution, or stop using it.
And honestly even with open source if the main devs stop updating or doing something you don't like you're time is almost certainly better spent doing what you're doing on a different editor or the old version then taking over development on yet another text editor.
So then why not just open-source it? Seems to me there are only drawbacks in keeping it closed.