Software without user interfaces is cool. This is about minimalism and “one thing well” and “keep it simple” and “less is more” and about less distraction: If we assume that software typically presents at least some information to the user, it follows that applications without real user interfaces will have to use UI elements which are already present, provided either by the operating system or by other running applications.
This should be beneficial to the user as he can continue using a frontend he knows and understands and as he doesn’t need to learn the shallow waters of yet another app’s interface [The lack of a “real” user interface does mean here that there isn’t a regular user interface presenting “productive” information to the user. Very often, though, there will be a need to have an accessory user interface, e.g. to define applications settings. Typically, this accessory UI is visible only once, at least if the application is well-designed].
Of course, in times of increasing iOSification and applification, it is not that easy to re-use existing user interfaces. Blending additional information in existing user interfaces means communication between applications (not to talk about method swizzling). This is not really facilitated by the Mac App Store Sandbox or by iOS. However, at the same time Apple introduces elements like the Mountain Lion Notification Center, and Growl is also available to sandboxed applications.
In theory, there has always been a strong sense of uniformity in OS X, visible in the not-that-obligatory-anymore Mac OS X Human Interface Guidelines, wanting to facilitate the use of widely-known, re-usable user interface elements, the counterpoint to this being the existence of quite a number of Mac App Store applications inventing the UI wheel again in a very “unique”, ugly way.
Technically, there should be ways and means to create applications resembling system services and without real user interfaces even in the era of the Sandbox. I will try to work on this in the near future, starting with Brow, a web browser bookmark syncing application that presents its information (bookmarks) not in its own, unique user interface (it doesn’t have a real, regular user interface), but in Spotlight. The idea should be extensible to other types of appilcations.