If there's a direct correlation between the software and the OS it runs on, then yes, it makes perfect sense that--at least by default--some level of parity should be created in the UIs.
If there's not a direct correlation, then it's a bit trickier. If your app runs on iOS5 and iOS7, they really are entirely different visual styles, but it's one app, so it's a bit of a toss-up.
Some things to consider with iOS, however:
- Most iOS users are on the latest version at any given time. iOS users tend to upgrade more than other OSes. This chart shows iOS penetration by version. As you can see, as of Apr 3, 2013, 87% of users are on the latest full version, and 98% are on the latest or one back.
- The iOS has a very consistent/standard look and feel, but apps do not necessarily share it. Lots of apps veer wildly from any 'norm' and seem to function just fine.
So one conclusion from that is that it's probably not a bad idea to update your App to match the latest OS, since such a large majority of users will upgrade. Another conclusion is that if your App doesn't mimic the OS to begin with, it's probably not a big deal either way.
So, some 6 months later or so I'm now neck deep in a mobile app redesign to bring things into alignment with iOS7. I'm getting more and more skeptical that that is the way to go. iOS7 was such a drastic change from the previous 6 versions that unless your app was relying solely on native UI elements in the first place, one needs to be careful about updating their own UI. I've begun to see a lot of apps get a quick 'updated for iOS7' update only to download them and realize it's pretty much the same UI as the app had before, but now there's this awkward over-use of Helvetica and blurred overlays.
So, my new opinion is, at least with iOS7: if your app has closely adhered to the native iOS UI from day one, yes, absolutely upgrade the UI to iOS7. In fact, if your app relied on native elements, this should be a rather trivial update.
If, however, your app had a fairly custom UI, be very selective in terms of what you decide to update. You don't want to end up with an Frankenstein-esque UI consisting of half your UI and half iOS7.