I like the "definition of arbitrary attributes" and I think the OpenStruct in ruby sometimes feels cleaner than using a hash, but I'm curious as to whether there are other specific advantages or use cases that make an OpenStruct a better choice than simply using a Hash.
I think this mostly comes down to a performance decision. From the Ruby Documentation:
An OpenStruct utilizes Ruby’s method lookup structure to and find and define the necessary methods for properties. This is accomplished through the method method_missing and define_method.
This should be a consideration if there is a concern about the performance of the objects that are created, as there is much more overhead in the setting of these properties compared to using a Hash or a Struct.
Additionally, something like a Hash has additional functionality with all of the methods it provides (has_key?, include?, etc.). The OpenStruct is a very simple object from that standpoint, but if you don't have any concerns from a performance standpoint and just want an easy object to work with, OpenStruct is a good choice.