Are questions about online backup services like:
on-topic for Super User?
As far as I'm concerned, I'd say no, they're off-topic, except for when the question is specifically about the software that comes with the service, such as, for example:
This is the reason why I've voted to close Online backup that keeps locally deleted files forever, as it's a shopping recommendation and just about the services in general.
I don't think this topic is specifically about the question, so discussing any other shortcomings of the question would only skew the results. Therefore this answer completely ignored the software recommendation aspect of the question. That's a completely different problem diamonds refuse to address in any conclusive way, and has no relevance here:
Either these services are generally on topic, in which case the same restrictions to the recommendation aspect apply like for general software questions, or it isn't, and we don't care about the ambiguous recommendation aspect anyway.
In the specified example, the service is very tightly coupled to the software that enabled the service. There is no service without the software, it's not even conceivable that there might be a website implementation for it! In addition, there is no real substitute to the service when intending to use the software: There is no software without the service.
The question is about the capabilities of a class of software products. Due to the inherent tight coupling of the server side parts also accessible from a web site and the client that runs as a real, standalone software on a user's computer, it doesn't make sense to separate them.
Only very few programs have this kind of coupling: Only online backup/syncing services and laptop anti-theft software come to mind. Everything else consists of more loosely coupled parts, where the desktop clients are optional (e.g. Twitter clients, desktop blogging software, etc.) and could easily be replaced while keeping the "cloud" component.
Since it is obvious that some parts of the service – the client software – is on topic, we should keep all of it on-topic to reduce user confusion: It's a combined product with a a desktop software component that is:
Therefore these questions should be on topic.
By all of it I mean questions about the functionality of the software, something that'd be on topic if there was no hosted server component, independent from whether they're implemented locally or on the server.
Consider the following questions (I'm using Dropbox as an example, as I'm somewhat familiar with it), which I consider to be on topic, but which appear to be off topic if we accepted @slhck's suggestion:
How do I access files I synced to Dropbox from a different computer? You go to the service's web site.
Does Dropbox store older versions, or do I lose them as soon as I save a newer file? has nothing to do with the client.
How can others access the files in my Public folder? This is implemented via the web site and completely unrelated to the client.
How can I send a link to a file in the Public folder? Until recently, the OS X client of Dropbox didn't have Copy public link. So this was achieved through the service's web site and completely unrelated to the client software.