As Yorrick says, in practical terms there are probably instances where Directors end up doing work for the Management Company as it's easier and sometimes cheaper than getting another supplier to do the work.
However, I do think unless it's properly handled, it runs the risk of being a conflict of interest.
In our Management Company, where it has been the case that we had to pay a Director for work, the invoices have been brought to the larger residents committee, discussed and authorised by a vote, without that person being present, in order to be transparent, independent and accountable.