What if someone registers an account 'Miscrosoft" and starts vandal editing? The media reports like 'Miscosoft blocked for vandalism in Wikipedia'" would be hardly better than 'Microsoft blocked on sight'.
Concerning the joint accounts I thought the main problem is that someone should be held responsible for the edits. I mean if there are some illegal edits done from this account and then someone claims it is not him, it is another user who uses the same account? On ru.wp we ban the joint accounts on sight, even though the company name policy has not been really enforced.
Cheers Yaroslav