Can it stand as the only tag? Probably not — because we want a platform tag and a domain tag on most questions. We have other tags that can't be used on their own, such as gratis and open-source. A tag that can't be used on its own is a red flag, but it doesn't automatically invalidate the tag.
Are there offline domain experts? Kinda. Not, I think, whole-domain experts, but experts in offline use of mobile platforms (so android offline experts, ios offline experts, etc.), for example people who spend a lot of time using their phone or tablet in airplanes.
Can I imagine someone subscribing to offline? Yes — people who are interested in using their mobile platform offline, and who might combine this with ignoring tags for the mobile platforms they don't use. offline in fact has one follower.
Can I imagine someone ignoring offline? No, I don't see why.
Is offline useful in a search? Yes, definitely. It's useful whenever you're looking for something that typically requires access to a server, such as maps and directions, editing data stored on a server, etc.
Is offline ambiguous? This is really what makes or break a tag. Yes, offline is somewhat ambiguous — we have people using it for anything that's offline (but it is not useful to tag every Windows/OSX/Linux question that way), whereas it should really be used only in cases where the offlineness is a non-obvious characteristic. But it isn't outright bad if offline is used in these obvious cases — the tag isn't ambiguous as in having multiple meanings, only as in being overused. To give an example of a similar problem, we have the same kind of misuse on Unix & Linux where distribution tags should only be used for problems that are specific to a distribution, but people tend to use them to mean “I'm running <distribution>”. That does not by any means invalidate the usefulness of these tags, we wouldn't want to remove them. We just remove the tag when it's used but not warranted.
Is offline a meta tag, i.e. a tag that characterizes the way the question is asked rather than what the question is about? No.
Conclusion: offline is a useful categorization, useful enough that we need a tag for it — if not “offline” then some other, less discoverable name. It gets some misuse but at a manageable level. Let's keep it.