You’re right, Kitely is not designed to put all worlds on a single grid like the Mainland of Second Life. And when that kind of concurrent world deployment is desirable or necessary, Kitely may not be the right choice of hosts. There are ways to simulate that kind of concurrency; for example, five people can start five Kitely worlds and hop between them instantly, but in the sense of walking from one world to another, Kitely doesn’t do that.
Even in Second Life, a large percentage of regions are private islands. You cannot see neighboring regions from those islands. So, the fact that all the regions are up and running concurrently really makes no practical difference - they are invisible. (Second Life does allow multiple regions to be joined into an estate allowing multi-region visibility; likewise Kitely has the capacity for megaregions with multiple region concurrency - visibility and access.)
Since my preference is for a private island, and since having a private island in Kitely offers the same level of immersion as a private island in Second Life, I find the Kitely service to be affordable and far more secure. Besides the fact that I can have a world up to 16 regions in size for a tiny fraction of the cost of a SL private island, the OAR download capability alone is worth its weight in prims.
There are some differences between Kitely features and the features available in Second Life, as well as other grids. As you mentioned, if you need chickens to be breeding while you’re away, Kitely will not work. I suspect there is a way to write breeding programs to simulate away time, but that’s probably beside your point. I suspect the issue is more about the sense and feeling one has about their virtual world. If we leave a virtual world running while no one is in it, it more literally mimics the real world that (thankfully) continues whether we are in it or not, and for some, that is a necessary aspect of feeling that their virtual world is real.
I would only add that it’s easy for me to accept both systems as valid implementations of whatever one means by the expression “virtual world”. In fact, the term “virtual world” has been challenged repeatedly in current writings on such things. “Immersive world” is probably more useful, but hasn’t fully caught on. In any event, it won’t be about the words we chose, but it will very much be about what activities we can perform. Kitely worlds can be used for such a wide-range of activities that fulfill the true effects of persistence and connectivity that it seems to me completely inaccurate to describe them as anything but virtual worlds.