I think a planer board 40' to the side is less likely to get motored over than an unweighted fly that is 50-60 yards behind the boat. In addition, unless some of the new superline is used, that length of monofilament would have lots of stretch in it.

Years ago I had a thing for trying to catch big trout on 2 lb test over in eastern WA. The line I picked had so much stretch in it that I had to resort to punching down the barb on the hook, sharpening it until it was like a needle and then holding on to the line and striking the fish 4 or 5 times just to make sure the hook was sunk in.

So... thought line length would really enter into being able to hook those trout.

Another thing that might be possible with a board is it would be easier to ensure that a lure went directly over any spot you wanted it to. With a long line you'd only be guessing where it was in relation to the back of the boat.