I did not see the previous post or replies so some of this may have already been said.
I see ceiling joist hung from roof rafters from time to time, and some installs work better than others. A number of times I got involved because there were cracks or deflections along the ceiling and prior explanations just didn't make sense.
The three 2xs along the tops of the ceiling joists I would call a strong back. I typically see a strong back when every other, or every third ceiling joist is hung, and the strongback is an attempt to carry the intermediate joists. Usually the strongback is only face nailed or toe nailed into the tops of the ceiling joists. With the nails loaded in tension, the nails withdraw and the intermediate ceilng joists deflect downward leading to cracked ceilings.
However, from this photo it appears that the vertical members could have been nailed to both the ceiling joist and rafter with the nails loaded in shear, and it appears that vertical members are present at every joist/rafter. The intended purpose of the strongback isn't clear to me, but it's only one photo.
Other things to consider is how well does the ceiling finish handle deflection? Sheet rock doesn't like a lot of deflection. The rafters could have enough strength to support the midspan of the ceiling, but maybe a decent snow load would cause some excessive deflection and cracking of the ceiling? Again, its only one photo, so I don't even know if this area sees snow or what the ceiling below is. You could do the analysis to check the capacity of the rafters, the connections, and work out a predicted deflection of the ceiling.
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