Several thoughts:
1. PROM is useful to check ROM if AROM is limited, to work joints of extremities with stabilized fractures when the patient is unable to actively move/unconscious, for joints at risk of contracture (you'll get more bang for your buck by teaching the nursing staff how to position the patient - having PT stretch the joints once a day is not turning up much by way of statistical significance in research), or when PROM is specified in the physician's order due to the nature of the injury (patellar fx come to mind).
Otherwise, PROM is rather a waste of the PT's skills. The immobile patient in ICU is frequently repositioned by the nursing staff for pressure relief and hygiene, so the joints are being moved regularly. In my trauma ICU practice, PROM is almost never indicated, because there are so many more helpful things I can be doing.
2. If the patient is conscious/able to cooperate, and the spine is not stabilized, AROM/isometrics within the spinal precautions is a terrific idea. I avoid extreme flexion angles at the proximal joints, based on my own nonscientific observation that I get a little flex and ext in the spine when I do that. I avoid resistive exercise.
3. If the spine is stabilized, surgically/orthotically/both, and there are no other medical or musculoskeletal contraindications to getting up, then up we get, ICU or not. Examples of medical contraindications would be unstable hemodynamics, patient on sedatives/paralytics and unable to participate, CSF leak with lumbar drain, etc. (Brain injury/CVA patients who are unable to participate do get the full upright treatment if OK with Neurosurg. A wise colleague refers to it as the Vertical Alarm Clock.)
4. Some surgeons have special precautions for some or all of their spine patients; these should be elucidated in the orders. For example, one practice operating in my hospital limits sitting time to 20 minutes every 2 hours; standing and walking are unlimited. Get to know your Neurosurg and Ortho/Spine docs and their preferences.
In fact, check out your ROM question with your surgeons; some may have idiosyncratic precaution preferences.
So, to sum it all up: PROM is literally the least you can do for the vast majority of your patients. Surgeon preferences for activity may vary (and may have no support in research). Get your ICU patients up! Hooray!