That's a consequence of having first class environments - but given that it's rarely used, shouldn't a sufficiently intelligent compiler be able to optimise it away in the common case? I'm sure I read a paper along those lines, but I can't find it again.
But seriously, every under-used feature harbors a ton of bugs in the compiler and drains resources for implementation. Promises, for example, seem like a "feature" in R that only really serves to destroy performance and no one uses it.
Agreed, but it's hard to know in advance what features will be useful, and once a language is established, it's hard to remove less-used features without breaking existing code.
On a related note, have you read the riposte paper? http://www.justintalbot.com/wp-content/uploads/2012/10/pact0...