I'm not sure I understand this part. When you say that someone with 30% in bonds might not have enough to weather a 2 year downturn, do you mean they might not have enough in cash to get by without selling some equities? If that's the case, it
is OK to sell equities in a downturn - as long as you don't sell all of them
Could you explain a bit further?
Yes, that's exactly what I mean - you can of course sell stocks in a downturn, and in fact, the retirement calculators assume that you do that (most assume you sell evenly across your portfolio), but every time you do that, you reduce the survivability of your portfolio since you're drawing down principal faster than planned.
My primary purpose for holding bonds is to minimize that possibility of having to sell stocks during a downturn, and to meet that goal, I need to have a certain amount (my 'belt-tightened' spending requirement per year times my guess at a reasonable maximum number of years for most market downturns). For me, there's no reason to hold any more than that amount.
Obviously my goals for holding bonds is not going to be the same as everyone else's, but that was sort of my original point - don't just go with some rule of thumb, figure out why you're holding bonds and calculate how much you need to satisfy your goals.
BTW, my apologies to the OP for going fairly off-topic. My on-topic feedback to the linked article would be that, although I agree with 100% equities, my risk tolerance is such that I prefer to have a small amount (which, for me, calculates to about 10%) in bonds. I, like the author of the post you linked, live off dividends and don't really need to sell other than to harvest capital gains, but you never know what life will throw at you and not having to sell stocks in a downturn is a big plus in my book.