The 1986 gestalts can all interchange their limbs, or what the Japanese called "Scramble". Metroplex was marketed/promoted in Japan as "Scramble City," and so he has connector ports that are compatible with the Scramble Gestalts. i.e. Metroplex is meant to be a city for their Scramble gestalt teams. The leaders of the Scramble gestalt teams can also connect to Metroplex in their base modes. Metroplex's weapons can also be held by the Scramble team leaders and all the Scramble gestalts (they all use the same peg size). Trypticon can also hook up with Scramble team leaders (in their base modes) too.
It was all a carry-over concept that was originally conceived for Diaclone but aborted when Diaclone was cancelled, so Takara adapted it for Transformers. The interactivity of the Scramble City w/ the Scramble gestalts was more prominently promoted in Japan -- and indeed there was also an OVA called "Transformers Scramble City" that showed off some of the Scramble gimmick thing between the gestalt teams. It's horribly animated and the story is about as awesome as Revenge of the Fallen, so you're not missing out on much. :rolleyes: But yeah, that's why. :)
Check out some of this G1 packaging -- not the "Scramble City" logo:
Encore Metroplex
Aerialbots (limbs)
Drag Strip
Trypticon