Re: Too much backing line or Too much fly line?
It's been done, but it's not common. One such reason for cutting the line back a bit, is when you have a very small reel with very limited space, mounted on a light outfit. If the known casting distances wouldn't exceed say forty feet and you happened to have a 100' long WF line, you might well determine that lopping off a portion of the running section ( rear section ) of the line would allow you to load it without rubbing the pillars.
Another application, some have cut double taper lines in half. Saving the other half for the day the first section is worn out, this two would allow the loading of a larger line on to a smaller reel. Again, it’s not a mainstream application, but it works for those who need an answer to a line capacity problem.
Where there's a need, there's a solution.
Itchmesir, beat me to it...