Airport Utility version 5.6 is not an old version. It was released at the same time as version 6.0. Apple's documentation regarding v5.6 is incorrect, inferring that it is only for older g Wi-Fi devices when in fact it is ALSO for n Wi-Fi devices.
On Mac OS X Lion, Airport Utility v5.6 is fully functional. v6.0 is not. That explains just about everything except the added iCloud setting in v6.0. If you find v6.0 to be inadequate, use version 5.6. It really is that simple.
Interesting points. The correct statement is more like: the AirPort Utility 5.x tree is now frozen, and I expect no more active development to occur in it, except for security fixes (which are few for AirPort Utility). The 6.0 tree is clearly the direction Apple plans, and where we will see evolution, which could include restoring some features removed from the 5.x version.
While you can argue 5.6 is "new," it is perhaps more precise to say that it's a compatibility update for a dead version of AirPort Utility to keep it functioning indefinitely under Lion, just as 5.5.3 appears to be the terminal release for Leopard and Snow Leopard.
Your definition of "fully functional" depends entirely on the features you need.
I do strongly agree that there is no reason to use 6.0 unless you find either the visualization of the network to be compelling or you need to link in Apple IDs linked to iCloud accounts.
I predict that v5.6 won't be the final version in the series unless Apple allows the v6 series to be more than a beta test, which is what I consider it to be. The uproar I have encountered in the Airport Utility threads up on Apple's Discussion board indicates that v6.0 is an enormous flop for anyone who wants and requires full functionality. I'm not aware of anyone who comprehends exactly why Apple hobbled v6.0. It clearly is not a replacement for the v5.x series. It is incapable of fully replacing v5.6. Instead, I consider v6.0 to be merely a novelty reminiscent of the iOS version of Airport Utility. If Apple starts to take the v6.x series seriously, then I'd believe it was ever intended to replace the v5.x series.
I spoke with Apple about AirPort Utility 6 on background, which means that I can use the information for guidance and understanding, but not quote directly from the conversation.
The guidance I received is that, unequivocally 6.0 is the way forward, and that the vast majority of Apple Wi-Fi users don't use the features that were removed.
The reason the 5.x tree remains and 5.6 was released, is clearly to provide legacy support for the edge users (in Apple's marketing view) who required features that the mass-market does not.
Apple has slowly removed itself from network and server infrastructure over the last several years. Remember that the original AirPort came in a PoE version with a Plenum rating? And the AirPort Management Utility (1.0 and never updated)?
The Xserve is gone. Leopard Server is a small-office add-on. AirPort Utility 6 is aimed at home users with no more than a handful of access points. It's pretty clear how that direction has been set.