RAT feeding EM GEN
BITE
The RAT will deploy if you have an "emergency in an emergency", namely to cover the extreme situation, where - on takeoff - you have an engine explosion that results in dual hyd line damages and subsequent leaking of Green + Yellow or Blue hyd fluid. The green HYD shutoff valve will close and the RAT automatically deploy to pressurize the remaining green fluid to ensure some flight control efficiency.
If - at the same time - you are in EM ELEC COND and your speed is below 260 KIAS the RAT efficiency is insufficient to both supply flight controls and EM GEN fully, and some elec bus shedding occurs. I believe this is the situation that you refer to, but this situation will probably not occur outside the terminal area.
In other words, the situation where you will have three generator failures + an engine explosion with separation and dual HYD loss in the middle of nowhere and be forced to slow down to low speed thereby loosing part of the NAV is highly unlikely. However, your concerns are shared by many pilots as the FCOM and the type conversion program does not emphasize the remoteness of this situation to occur.
As this explanation is purely from memory, do correct me if there are more likely scenarios where you end up on "RAT only" outside radar coverage. We all know that "S**t happens" and unexpected system interface ruins the day!
Out of curiousity, which airlines do not have the DDRMI installed? It is not described as "optional equipment" by AI.
Capt T.