In my experience, if you're in a real silt out, you see it coming at the very least, seconds in advance and know to start getting near the line. The worst I've had was a small passage where my buddies argon bottle got tangled behind me, and the passage was siphoning so I got somewhat blindsided. A 30 second pause (I grabbed the ceiling to avoid "drifting") and viz was back up to a few ft, the line was quickly found, and we were on our way. It was easy to find it because I had seen passages like this before, and knew how fast they could get nasty, so I was much closer to the line than in let's say, the gallery or pothole line.
Here's an example video I took last weekend-
Minor siltout... on Vimeo
You can see that even in what most would consider a "complete silt out", there's still 2-3ft viz other than around the 40 second mark where it goes down a bit. That's quite a bit of viz in a small passage, especially with even mediocre line and situational awareness. Furthermore, we knew before going down this passage what we were getting into, so you can see that we return to a completely full stage bottle as a "safety".
I don't believe that spinning a student simulates a real life scenario, and if it does, there's more important issues to deal with other than lost line drills. I also don't believe it's an entirely bad thing, either. Both methods prove the point that the only good way to survive a silt out is to know it's coming. I took the lost line drill as more of a lesson on "here's why you don't get yourself into this situation" rather than "here's how to get out".