This vid they don't seem to dissolve in
That was one of the many complaints about doom 3, everything seemed too sterile with the way the enemies burned up. Hopefully they stay around for at least a while.
How can they? Look at crysis 2 there is so little memory that you see ground debris being drawn in and popping up a short distance in front of you. With the best will in the world im sure they thought of it but it will have be been cut during the optimisation process.