This would be a very good idea; I proposed this change on GitHub a few months ago, but it didn’t get a lot of traction: https://github.com/rails/rails/issues/5545.
I think chaining scopes with OR would be a very common use case for a search functionality. For example, you could allow users to search for something matching all parameters (chaining with AND) or only one of them (chaining with OR). I think it would be a relatively simple change to implement, but I’m not at all familiar with Rails’ source code, so I don’t really know what I’m talking about here.
Anyway, consider this +1 from me.