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.