We use propofol and fentanyl routinely, and Precedex alternatively and for weaning. However, studies have shown there are some adverse effects from longer-term propofol use. (I'm in class right now, though, so pardon me for not taking the time to find and post a link!
). I am inclined to think your medical director may have a point, though...we sedate vents pretty decently, and I have noticed quite a few patients recently with difficulty weaning off the vent, and they end up going for trachs. I know there are multiple factors involved in failed weans, but I need to research more the role of sedation in these instances. As you said, OP, we don't want folks bucking the vent and freaking out, but you don't want them totally snowed, either. That "happy medium" is often easier said than done, though.
I will say that sedation is always very individual. There are those little old folks where a tiny bit of fentanyl knocks them out, and then there are patients maxed out on everything and still kicking and trying to self-extubate (often these folks have a history of drug abuse or ETOH, so I sometimes think all the sedation in the world wouldn't cut it). Looking forward to seeing others' comments and experiences.