From SO:

OpenStruct objects are useful when you need something to fit a certain method call interface (i.e. send in a duck-typed object responding to #name and #value ), or when you want to encapsulate the implementation details, but also want to avoid over-engineering the solution. They also make an awesome stub object, and I often use them in place of framework stubs when I don’t need the overhead of a stub/mock.

You’ll just have to get over your dislike of Ruby and write a few apps in it ODL - just so you can see why so many people love it

Same. I’ve never used Ruby Structs… I learn about them like a week ago in this post from Alchemist up above.

Also never encountered it in any codebase so far…

True!

Finally, we have OpenStruct which is part of Ruby Core as well. At this point, you might be thinking: “Hey, looks like an OpenStruct is better in terms of melding Struct and Classsyntax and functionality.” Well, you’d be very wrong in terms of performance but, as mentioned with Class usage, I promise to expand upon this more later.

Warming up --------------------------------------

               Array     2.000  i/100ms
                Hash     1.000  i/100ms
              Struct     1.000  i/100ms
          OpenStruct     1.000  i/100ms
               Class     1.000  I/100ms

Calculating -------------------------------------

               Array     23.854  (± 0.0%) i/s -    120.000  in   5.030763s
                Hash      8.908  (±11.2%) i/s -     45.000  in   5.072652s
              Struct      4.798  (± 0.0%) i/s -     24.000  in   5.005263s
          OpenStruct      0.178  (± 0.0%) i/s -      1.000  in   5.618325s
               Class      4.230  (± 0.0%) i/s -     22.000  in   5.203019s

Comparison:

               Array:       23.9 i/s
                Hash:        8.9 i/s - 2.68x  (± 0.00) slower
              Struct:        4.8 i/s - 4.97x  (± 0.00) slower
               Class:        4.2 i/s - 5.64x  (± 0.00) slower
          OpenStruct:        0.2 i/s - 134.02x  (± 0.00) slower

Still it seems that Structs are faster, but if it was, why is not idiomatic to use them ?
And use a class for “singleton instance” and struct for the rest like in Swift.