In a perfect world, fresh fuel whenever you need it is a wonderful thing. However, the whole purpose of having a backup generator is to be able to use it during less than perfect times. To have a generator and not maintain a reasonable amount of diesel fuel storage is certainly not the wisest course. Additives and careful storage really do work.
Yes, fresh fuel is a good thing. But old crystalizing fuel is worse than useless even in perfect times. I understand that fuel stabilizing agents can stretch this out... really I do.
But it still remains that there is a balance maintained between the advantage of refreshing fuel stores at some reasonable frequency and trying to store as much fuel as possible for as long as possible.
The larger the fuel store, the greater the magnitude of the storage management problem, and of course, the greater the cost of management.