One DOFF that's missing is the Emergency Medical Hologram. They can be used in a space slot to heal crew.
My VA doesn't use the DOFF system much except to pick up additional dilithium. Once my characters has the DOFFs they need for their space and ground slots, they'll slack off also. I'll just run a few DOFF assignments as a residual source of dilithium.
I was extremely happy with the rewards for participating on the Tribble Test Server. I received all of the available bonus items. The only sad part was that I could not keep all the cool DOFFs that I had collected. :-P
Although there are some DOFF chain assignments, I think the system could be made more interesting. Perhaps succeeding in some DOFF assignments would open up special missions for your character.
One nice benefits of the DOFF system is the ability to recruit Bridge Officers. My characters have received several purple BO's and a couple of green saurians with the efficient skill. Some of the BO's find their way in my character's bridge or away team. On that note, if anyone in the fleet need BO's just send me a note with your needs. I often refuse or sell BO's just to make room for additional ones. Strangely, I sometimes find BO's in my character's inventory. I tried putting some into the fleet bank but that didn't work.