I'm alsol struggling with your question.
Right now, my decision depends on a lot of things. If I'm fishing shallow, faster water and I'm using a tight-line fishing technique - Brooks/Czech method, direct upstream nymphing - I don't use an indicator. When I'm fishing deeper water and I don't think I'll see a strike and/or I'm using slack line techniques - high sticking - I'll use an indicator.
One problem I now realize I've been having is that I've been using a lot of slack-line casts and I've had so much slack in my leaders that often when I got a strike I didn't know it. Also, in faster, shallower water I've been too concerned with drag. Now, I believe that often trout don't have time to inspect a nymph that dragging slightly too fast, so I'll go with tight-line nymphing and not use an indicator, especially when I want to my nymph to mimic an insect rising to the surface.
I find floating indicators very helpful when I want to prevent my fly from getting hung up on the bottom. (The nymph should be close to the bottom but not on the bottom.) Also, when I'm moving around on a stream and fishing water of varying depths I simply move the indicator. I think that's much easier than adding or subtracting weight from my leader.
This previous post might be helpful:
http://www.flyanglersonline.com/bb/s...hingamabobbers
Finally, I believe sometimes there is no clear-cut answer.
Randy