I believe that telltales are only necessary until you know your sail and settings and thereafter are un-necessary and even possibly a distraction.
Guys like Asby have so many hours on the water, they know what works and what doesn't and they simply trim by experience and feel.

For us duffers, until we reach that level of excellence, I'm afraid we are stuck with streamers <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />