I've recently been playing through the Doom 3: BFG Edition, which added a shoulder-mounted flashlight instead of forcing players to switch from flashlight to weapon. This "corrected" what many people viewed as one of the huge problems with Doom 3. There were many jokes about there "not being any duct-tape" on Mars and mods appeared shortly after release that added a flashlight to many of the weapons.
To be honest, though, the flashlight/gun switching was something that never bothered me about the original Doom 3. While yes, it does seem kind of strange that none of the weapons have lights, I've never gotten the argument that the doomguy should have duct taped the flashlight to one of his weapons. Under the suspension of disbelief that he can carry a pistol, 5 two-handed weapons and a chainsaw all at the same time, I think it makes sense to not want to duct-tape the light to only one of the weapons. The flashlight is small and can easily be used and put away while still allowing quick access to any of the other weapons. Strapping it to any of the other guns would remove this property.
I guess it all comes down to what breaks your suspension of disbelief. For me, the flashlight never did, and while playing the BFG Edition I miss being able to bash corpses into nothingness rather than waste pistol ammo on them.
