The motivation to decompile (partially or fully) a game comes from the possibility to find some hidden rules and secrets or simply to understand the internal logic of the game (like the AI, the algorithms, ...).
For example, i've always wondered what was the combat system in Phantasie or the AI of Colonial Conquest.
I've both played Mercenary 1 & 2 (very interesting), but i didn't find any big reason that will push me to decompile one of these games. The only thing the decompilation could add to the current knowledge of these games is the exact mapping of the areas, but when i've played Mercenary, i don't remember having missed anything or let a place undiscovered. Do you ?