But, as the creation date of the files are important to me as I use it for other purposes outside the MC environment, couldn't MC be instructed to report that date in the creation file field again when any tag changes rebuild the file?
Or I'm asking for an impossible thing?
I'm not 100% sure I understand what you're asking for here, but...
If MC re-writes the entire file, which is required for some container formats, then as far as OSX is concerned, it IS a brand new file. MC doesn't directly set the Created or Modified Dates on the Filesystem. That's handled by the filesystem and the OS.
If what you're asking for is for MC to "cheat" and manually change the filesystem Date Created to match its own database, then I don't know if this would be allowed, but it would certainly be "bad practice". The filesystem dates should remain accurate for what they are in almost all cases (except when restoring from backup).
If you're asking for MC to track, internally, the original Date Created, regardless of what the filesystem says, then I think the best solution for this is to create a custom "Original Date" field and copy the information from the files to that field when they're imported (Tag on Import can automate this). MC's internal [Date Created] and [Date Modified] should always reflect the same date as is reported by the filesystem.