I've never heard of a fly line breaking before the tippet, leader, or a knot. So what does the fly line strength really matter?
The only fly line I've ever had that was shortened was because somehow I put a knot in a 4 wt line I have without realizing it about 15' from the tip end. I still can't figure out how that happened, but by the time I found it, it was so badly knotted I had to cut the fly line off at that point. It's just about the most puzzling thing I've ever seen in 25+ years of flyfishing and hundreds of thousands of casts. Other than that, every fly line I've ever owned has stayed whole, other than one that found a propeller.
My passion for catching fish is eclipsed only by the fish's passion not to be caught.