Mark,
I understand and appreciate what you are saying - it is possible for a 5 strike put in an extreme case for example to quote at 4.5/5.5 and you could find yourself at a point of $-50 in your account if you had zero cash after the put sell, etc.
I do agree with you that in a scenario such as this, the covered call won't give a person any advantage over the put write, as the call spread would likely be the same and the person would still find themselves in a $50 hole.
It isn't just net short positions that can go negative however - even a bull call spread can go negative. For example, if you buy a 80 call, sell a 90 call for say $150, now the stock plunges to $10 a share, they might price out such as:
80 call - bid .05 / ask .25
90 call - bid .05 / ask .20
so, to close, you'd get .05 for your 80, but have to pay .20 for your 90. I've actually had Bull Call Spreads go negative on me. However, a person in this situation also knows that in reality, both sides will expire and they won't have to buy the position back. Long calendar spreads can also go negative.
I have never had a cash account to go negative, but I suppose it could happen and I will believe you that the broker wouldn't allow it to remain so. Personally, I would never go to the absolute edge of what I could sell anyways. For example, if I had $4000 in an cash account, I would sell at most a 35 put (quite possibly less) - since it's always good to have some extra money there for adjustments, etc. anyways.
So, there is really nothing big new here IMO - If we're all going to worry about almost impossible scenarios, I think we should just go and buy lottery tickets. Actually I think a more worrysome item realistically is like what apparently happened to that one poster - he was long some GOOG calls, they were out of the money but he didn't close them, they went in the money just barely at expiration and they were automatically exercised for him, even though his account didn't have the money for all those shares.
IMO - The automatic exercise is not a good thing - the call or put holder only should have the discretion of exercising at expiration - it should not be done automatically just because some stupid people forgot to ever do anything with their in the money options.
JJacksET4