First a caveat: I'm an 
utter newbie at SE4, really just been fooling around with things for a bit so far.  But I've noted at least 
one eventual use for single-facility-space worlds, 
if they are moons of a larger world.  
 
  
For those tiny, one-space (airless?) moons of an otherwise-potentially-useful world, consider Fleet or Ship training facilities.  As far as I have been able to tell, while only one (say) Ship Training facility per 
world is useful, in the event a sector has multiple worlds ... the effects 
do stack (one per world, not one per sector, see?).  Got a planet with two moons?  One ship trainer per moon, and a ship trainer on the planet itself ... and 
*poof* instant +3%, +6%, or even +9% net training per turn (depending on the level of the facilities you can/do build, of course).
Once you have level III facilities, put one of each on the planet and a fleet centre on one moon, a ship centre on the other (totalling two facilities of each type, in that sector).  +6% fleet / +6% ship training per turn.  With Psychic technology, and each level III psychic training facility available somewhere in that system, that becomes 9% apiece, per turn; two turns to near-maximum training (sans combat experience, ofc) ... all with two single-space moons, and a two-space planet.  More than that, is IMO pure gravy.
With Atmosphere ConVersion technology, you can eventually manage 9% (12% with Psychic technology) per-turn training in 
each of Fleets and Ships 
anywhere you have two small moons around one larger planet (~5 spaces apiece is 
all it takes) -- 
and three shipyards on top of each other.