it makes sense for his spare body to never show up in-game but. BUT. it's fun to speculate. my two favorite options are:

1) he sustained some critical damage prior to the main story, and the spare body had to be either butchered for internal organs or used entirely. i also like to think sam did it, as a big fuck you to world marshal/desperado budget... more fun for my depressed boy and another scenario to throw sundowner into a meatgrinder, just how i like him. he'd find it amusing anyway.

2) sun is resourceful, so it was meant to be used after raiden's mmm visit. which leads to more tasty details. first, he expected bad damage from the get-go. second, he could just throw this ai controlled chassis at raiden to wear him down and finish him off later. so he doesn't really care about killing raiden, just giving him enough hell and action. and i don't think he cared about his own well-being, too. not in a reasonable, human "i need to set up all the soft padding/financial backup" way. more like "this will be my new skin once i shed this one, rub it off on rocks RUB IT OFF".

i'm always trying to pin point this feature in him. yes, he's an entrepreneur by nature. quick on his feet and intelligent, thanks to having no inner dialogue at all. no doubts, no ego. probably what helped him survive for so long AND led to such a fiery crash for desperado at the same time. big hollow reptile to me ↓