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It's important to remember that other implementations treat CPython as
the "gold standard" for compatibility purposes. If we declare
something to be an ordinary SyntaxError, then that carries strong
implications for what other implementations should do.
Some kinds of errors are inherently implementation specific
(MemoryError and SystemError spring to mind). No sane implementor is
going to try to match CPython like-for-like when it comes to those.
SyntaxError, however, is typically defined by the language definition,
not the CPython implementation of it. By introducing a new exception
type, we're explicitly telling other implementations "Look, this is
our problem that we don't plan to fix as it doesn't typically arise in
real code, but please don't deliberately cripple your own
implementation just to match this behaviour". I'm strongly with MvL on
this one - SyntaxError itself should never be raised for legal Python
code just because the CPython bytecode generation toolchain isn't able
to handle it properly.