What you say makes sense. I can see the benefit in message passing as a first class citizen so it allows extraction of some processes to a different node. But you still have to manage the process placements.
Erlang does have some tools that make it legitimately easier to write cluster-aware software, such as the message-passing operator not caring about what node the target is on, and transparently handling all network communication from serializing the Erlang term on one side and deserializing it on the other. Erlang's terms, the basic data types, are designed for exactly this use case, and as such a first-class concern, they are good at that particular use case.
However it's still easy to accidentally write some code that will only run on a single node and you still need to be aware of what you're doing to be sure you don't accidentally wire in a hard-coded dependency on being in a single OS process. But when you contrast that to the default state of most other programming languages, which is that they have essentially no concept of cluster communication at all, you can still see how this is an advantage over those other languages.
Erlang/Elixir is amazing because it brings so much good things and they are not that difficult to learn with some experience with functional programming.
On the plus side, out of the box it can do things like "run this process N times", "don't run these on the same node", "run this on every node exactly once".
I'd like to get some experience on a real distributed project in Erlang. Mostly in order to be able to assess how big a productivity booster it really is. :-) But it's a chicken and egg problem. Not a hugely popular language <-> fewer developers <-> higher risk for the company.
At Basho, we hired developers to work on Riak who had no Erlang experience. It's a remarkably simple language, syntactically, and the lack of types makes it easier to get up to speed.
(There are, of course, countless other challenges that it brings, where an experienced Erlang person or three can make a big difference.)
Cheers to all you Basho/Riak folks. I have worked with one before!