Two things come to mind.
First, VIPs can be Scientists and Engineers (or Captured Soldiers!), so depending on where in the codebase you're looking, you might be dealing with an Engineer who hasn't been transferred back to the strategic game yet.
Secondly, there are references in the code to both Engineers and Scientists "leveling up" (to increased effectiveness), though it doesn't appear that any of this made it to the released game. Depending on how the Docs were expected to gain XP (sending them out on missions with your squad, perhaps?), there may have been a time when checking for non-dead engineers made sense, similar to how the game checks for non-dead and non-captured soldiers when filling the Armory.