by that moment it should be clear that what they can rely on and what they cannot
The only difference if the implementation is the spec is how they know what they can rely on. If the implementation is the spec, they decide by reading the source; if it's a document writen in English, they decide by reading that.
Implementation can be, and will be, changed, inevitably. Then does the language change as well, or the languages remains the same but just implementation is improved? How can you tell the difference purely from the source?
Some Scheme implementation evaluates arguments left to right. You can see that by reading the source. In future, it may switch it right to left, maybe for better optimization. The spec in natural language, or more formal and abstract form like in Appendix A of R6RS, can explicitly say the order of evaluation is unspecified. How you tell your users that they should not rely on the evaluation order purely by the source code, given the state of most programming languages?
Ideally I like to think the source only describes the spec and the compiler and runtime figure out the details, so maybe spec-by-source and spec-by-other-notation will converge in future. Is that what you are talking?
(Please don't argue that unspecified evaluation order is bad or not; I just use that example to illustrate the point. And incidentally, since Arc is defined in terms of Scheme, the order of argument evaluation order is just unspecified as well. But it's just delegating the spec to a lower level.)
One point everybody else is missing: since arc explicitly makes no claims of backwards compatibility, the notion of a spec is meaningless.
If the goal of a language is to be readable there's nothing wrong in the implementation being the spec. Consider it a form of self-hosting, or of eating your own dogfood.
An implementation in a reasonably high-level declarative language is a more reasonable 'spec' than one in C. More features are amenable to checking just by reading rather than by execution.
When something is obviously a bug it doesn't matter if it's a bug in the spec or the implementation.
Those two categories -- obvious bugs, and questions about what is or is not in the language that should be answered by reading the code rather than executing it -- seem to cover all the objections people have raised.
At least I'm talking about the attitude of spec-by-source in general, not particularly about Arc, FYI.
Edit: I agree that more abstract, declarative language is closer to spec-by-source. If somebody says Prelude.hs is the spec of Haskell's standard library I agree. But the core language semantics is still not in Haskell itself, is it? (I'm still learning. Correct me if I'm wrong.)