It is appropriate to edit the original question and broaden its scope from Mac OS X-only to any application that meets the non-OS-specific criteria? It seems like the broader question should attract more canonical answers that cover the waterfront of Markdown editing options irrespective of OS, which ultimately seems like it will be a more useful artifact to leave behind for others.
It depends probably more on the answers than the question. Questions should only be broadened in scope if the answers that are given, or can be given, are equally applicable.
This requires that the user still gets the same, good answers he'd have gotten if the question were unchanged. That's why users add their particular situation (e.g. OS used, or specific requirements) after all: to get answers tailored for their needs instead of generally good but not applicable answers.
Sometimes, parts of these stated requirements are irrelevant to the question, and the user is unaware of that fact. It probably doesn't matter what OS you use when discussing the general advantages of SSDs. If you're knowledgable about that topic (and whether OS makes any difference), and you agree, just remove OS references, as they don't matter. Users looking for cross-platform tools also make this mistake: Tools are very often available for Windows, Linux and Mac OS X, so questions looking for a tool that works on two of these OSes are often just as applicable on other, similar systems.
It's also a good idea sometimes to broaden the scope to prevent questions from being off topic or too localized (the former being easier to determine, and fix, than the latter). If someone asks about web-based Markdown editors, you can either make it desktop Markdown editors or wait for it to be closed.
Other times, specific information in a question is essential to get good answers. An obviously completely non-applicable answer got eight up votes (now at +8/-3) before an answer that actually addressed the user's situation was posted. Now, imagine this question wouldn't have been tagged osx, but e.g. unix. Broadening the scope is only useful if the user's problem is still being addressed. In this particular case, it's doubtful, due to the relative popularity of Linux and OS X on this site.
Going back to the SSD example, Apple still hasn't managed to support TRIM on third party SSD drives. Any question about SSD maintenance and correct use, cleanup, etc. must therefore retain the OS tag.
There are is the issue of users using tags to navigate this site. If you're looking for questions or answers relating to ubuntu, and the question isn't tagged, you won't see it. The missing osx would certainly have prevented me from answering the question linked above. Why research an answer if the user is on a different system anyway?
This leads to another problem created by broadening the scope: If you spend quite a bit of time to research and write a good answer you have a certain expectation of being rewarded for it. This is only possible if you're actually addressing the user's question. But the user will probably accept the It's not possible on Windows answer, completely ignoring your three page (Linux) C source code that compiles into a utility doing what the user requires*. And you posted just because someone broadened the scope...
Another issue is that, if the only possible answer is negative, e.g. It's not possible or This doesn't exist (yet), it just won't be posted. Users will post answers that don't interest the OP, and he'll never get the information that it's not possible in his particular situation.
It's not a good idea to edit a question, broadening the scope, only to have it be closed as exact duplicate or NARQ. Many questions relating e.g. to backups could fall into this trap, with only specific requirements differentiating these questions from their more widely applicable cousins.
Moderators have the amazing superpower to merge questions. If there is a situation with multiple (useful) questions, that aren't duplicates but can be merged and still retain their usefulness, ask about it here on Meta, flag the question, or ask a diamond moderator in chat.
If the topic is important enough, create a community wiki FAQ question like @slhck did with this one and close everything else as duplicates, although we're still debating how to handle some of these topics.
Your specific example suffers from being a software recommendation question which is generally considered off topic on this site, although the FAQ doesn't state it at all. So instead of three copies of this question (one per popular OS), or one question (the unified all-encompassing), we'd probably prefer 0.
If you know a good related answer, just post it as a comment to the question. I've done this before, "On OS X, you could do this or that" — it doesn't create a non-applicable answer, but could still help someone looking e.g. for solutions that work cross-platform.
If, like in the linked topic, some answers work in a broader scope than the question has, just mention it.
* Don't laugh. I've done this. Was a complete waste of time, for a different reason though.