This rather blurs the line between the fundamental difference between symbols and strings, huh? The whole point of symbols is that you could replace every symbol, say :foo with another one, say :bar and your code would function identically. So def foo would also become def bar and any calls to it as well. Think of a symbol as "identifier number 1" where its specific value is irrelevant, where as a string its specific value is the thing that matters, say for example in an http request to another application. All of this indifferentness introduced to the Ruby community is kind-of eliminating the need for symbols at all in the Ruby language. Not saying it's good or bad, just pointing it out. Their entire purpose for existing has now become moot. (also frozen string literals have reinforced their coming obsolescence)

@b264 String hash keys are automatically frozen, and have been for quite some time:

$ ruby --version
ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-darwin16.5.0]
$ ruby -e 'puts Hash[?a=>1].keys.first.frozen?'
true

So in practice, string and symbol hash keys are functionally equivalent, and I believe they perform just as well as each other.

This particular "problem" that Sinatra tries to solve with IndifferentHash (and Rails solves with HashWithIndifferentAccess) is due to Rack always using string keys to store the parsed query parameter string. Most Rubyists prefer to use symbol keys in their code, so this pattern of casting symbols to strings has developed over time. In a perfect world, Rack would (optionally?) use symbol keys for this data structure, but I guess that would break too many things.

I'd say it's fairly limited to a handful of use cases—mostly this one—so I don't see it as endemic, but that's just my perception. Most Rubyists I've observed seem to grok symbols and prefer to use them wherever possible.

it's fixed on 2.0.0.rc2 but not working at all on 2.0.0.rc6.
also report the issue here padrino/padrino-framework#2132 (comment)

hash problems accessing hash in post request through params as described
here https://stackoverflow.com/questions/14540729/how-do-i-map-an-application-json-post-request-to-the-params-in-rails