Tim, the tinsel is really a matter of preference and can be used either wide or narrow for specific hatches or circumstances.
Take a look at this TroutNut photo of a Beaverkill River March Brown. See the banding along the body. A pretty good match with wider tinsel.
- Maccaffertium Beaverkill MB.jpg (85.85 KiB) Viewed 8597 times
Also, on sunnier days it is often to the fisherman's advantage to have a more "flashy" pattern that will pick up the light and reflect it about as the imitation travels downstream. It mimics attractive properties and movement.
Also, the flymph stage represents that emerging stage of an insect's life as it works to the surface to shed its nymphal shuck and promote its wings to the world above. Many insects use air bubbles to aid in this process. The "flash" of the tinsel can aid in that translucent mimicry of air bubbles.
I don't use the wide tinsel a lot on my patterns. But almost all my march brown patterns do and I always have a few just for the purposes listed above.
I use copper wire a lot more to add just a little bit of flash, but mostly for durability. The wire id definitely easier to work with. The tinsel can be a bit tricky at times, especially the wider tinsel, as it wants to slip around.
I came across it first in Dave's 1987 book 
