Oh thank you - It may well be how is was decided some time ago that NUMBERS was going to skip XML support because it was impossible to completely and flawlessly emulate Excel's XML behaviors.
After all, as many of Microsoft's apps have been created to be "incompatible by design", they preclude being emulated - at least not without going to a lot of time / trouble / expense.
But - there's the "OPEN OFFICE" app, "Calc" which performs some subset of Excel functionality. The pragmatist in me says, "Most people are going to be creating relatively simple spreadsheets which do not make use of secret-sauce, corner-case, MS Excel behaviors. Do we need to be saying "It's all or nothing" - if Excel compatibility is the reason for no XML I/O inNUMBERS?
The folks at Apple already know everything that NUMBERS does. If there were simply the ability to import and export NUMBERS-compatible-XML, that would be a HUGE leg-up to being able to use NUMBERS in various doc-processing workflows.
Once NUMBERS had support for XML, it would be possible for other apps, running on iOS, for instance, to be able to import/export data - all of which could be later be amalgamated via NUMBERS.
If anyone has any other compelling ideas for why it would be good to have XML support in NUMBERS, would you please make a note to this thread and submit a feature request to Apple's BugReporter.