The purpose is to be able to use the amount generator during indexation.
At some point, it executes:

        if amount.getQuantityUnit():
        ...
        for x in property_dict.iteritems():
          amount._setProperty(*x)

where `amount.getQuantityUnit()` may getResource before it is set.
Any further access to the resource category would be wrong.

There may be a way to only change the amount generator but this kind
of pitfall is likely to happen in many other places if we keep such
a read-only transaction cache for new or temp objects.

See merge request nexedi/erp5!1818