I don't think I have been with an emergency supply that was not three phase although I had one with a battery supply which switched the three wire AC lighting into one two wire DC battery supply (not VG when fluorescent lights had been added to the lighting network!).
If you take the 'old' list of equipment that must be capable of being supplied from the emergency supply it is true that to some that meant 'only' the emergency supply - we had an amusing but expensive exhibition of how that, when applied to the rudder angle indicator, could precipitate excitments.
On more 'sophisticated' ships the reverse mistake was made. Anything is more secure if run from the emergency supply. But when, for instances running steering gear telemotor pumps from a supply not also capable of supplying the steering gear pumps themselves or when, similarly, supplying the thruster controls from the emergency supply then in both the possibility (actuality) of crippling the ship occurs in the event of an emergency supply (only) outage that would have otherwise left the vessel seaworthy. On the knockings of my career we also finally worked out why the standby generator would not start for the surveyor during a routine docking. The luboil priming pumps were fed from the emergency supply and the primed luboil pressure was interlocked with the start-up. The emergency generator was secured for surveys of its own again making outage of the emergency switchboard a potential 'fatal' single failure mode.
I am happy to say that current rules mean, in simple terms, that such essentials now 'find' an available supply if there is one.
In general terms three or single phase is decided by consumption but I would not like to rely on that criterion alone as lots of little loads might end up on one phase over time leading to unbalance. In the same way that houses are fed from alternate phase with neutral down the street (in an urban supply) unbalance is better avoided. I cannot immediately think of a 440V single phase load and to run the whatever from the domestic supply (main or emergency) usually introduces a set of transformers into the FMEA.
(Sorry - this is a hobby horse of mine, or was).