For me, it just adds a bit of frustration. Was there any other purpose of this "feature"?
Why do unfinished buildings degrade with time?
Probably to discourage their use as mass storage. The more important reason is that otherwise your ACU gets blocked by 0.01% completed factories or AA turrets and you get cancer, it's already too toxic with the current decay rate.
I think much more so the latter than the former. The decay rate is so slow that the amount of mass (or energy) you lose per tick is very low compared to that stored for a nearly complete building.
For example an unfinished T1 air factory can actually store quite a bit of energy to help get a gun upgrade sooner for example.