EMB170 experiences severe turbulence during descent into Denver; then the First Officer has a discussion with the Captain over whether a log book entry should be made for the encounter.

Narrative

Our flight entered severe turbulence between roughly 16;000 and 13;000 FT (I am not sure of the exact altitude as I had my hands full flying the aircraft). I was pilot flying in the right seat and we encountered downdrafts that were showing -2;000 FPM and +/- 30 KIAS; if not more through our altitude with the autopilot on. I used control wheel steering a few times and once clicked the autopilot off to control it. The Captain reported severe turbulence 2 times (possibly 3) that I recall. The PIREP was: APA UUV / OV DEN200030/TM XXXX/FL150/TP XXXX/TB MOD-SEV 150-130/RM. +/- 40 KT IAS. I strongly suggested 4 times to Captain that it needed to be written up as severe turbulence once we got on the ground; then [brought it up] one more time on the way off the aircraft. He stated; Maintenance wouldn't do anything about it and would say it was only moderate turbulence. I stated; 'It did not matter; you reported it and it should be written up.' I left the aircraft and decided to discuss it with Dispatch. I spoke to one Dispatcher; the Coordinator; and Maintenance. I told them all I was First Officer on this flight and did not want to step on any toes; but the Captain reported severe turbulence 2-3 times and I felt the aircraft should have a severe turbulence write-up. Maintenance told me the Captain did call and reported as well as wrote-up moderate turbulence. I stated; 'There is no write-up for moderate turbulence.' Maintenance told me [they] were on their way to the aircraft to check it out and it would be taken care of. I felt I did what I should as a crew member without getting confrontational.