Regardless of the strength of an Elemental, only one of each type can be brought into existence during any "day". Thus, if a character possessed a device to call up an Air Elemental, but before he could employ it an opponent conjured an Air Elemental, another could not be created until the next day. (Monsters & Treasure, p18)Sounds like a dissociated mechanic, doesn't it?
Almost. Except that I think it actually arose from the idea that an elemental was a creation rather than a being summoned from elsewhere. The Conjure Elemental spell caused a substantial quantity of matter to take on a crude intelligence and independent behavior. Thus, once the earth or air in an area has been used, it can't be used again; you have to conjure a different kind of elemental. So, it's actually a very associated mechanic, treating matter in the area as a limited resource.
This was before the elemental planes were conceived, remember. You can see this in the Invisible Stalker description that comes immediately before the entry for elementals: stalkers aren't considered air elementals yet, and they are said to come from a non-dimensional plane.
The elementals in The Fantasy Trip have this same feel of being created rather than summoned. Both TFt and the LBBs fit my "plane-less cosmology", although the LBBs do have these suggestions of "extradimensional space" that doesn't have a clear concept yet. I'm actually OK with that: an endless number of "pockets" outside of normal space and time that are vague and difficult to comprehend.