Â If you can see the ends hit the ground or a big ledge, or are familair with the route, have a need for haste, can all be reasons not to knot... Â
Reasons yes, but not very good. Murphy's law applies here too.
Assume that the very thing that makes you want to wear a helmet, the falling block, flying beer bottle, fist size stone , #4 cam or anything else on a colision course with your noggin, hits you and only startles you, how far would you have to travel down that rope and for how long before the rope becomes impossible to grab? if you are not knocked out, that is...
Unless you are using an auto block system, tied or mechanical, those knots are just about the only thing that could save your scorny butt from a free fall.
The big ledge is now a very unfriendly trampoline, just something to bounce off... the familiar route develops an unfriendly aspect.
This argument is about the same as asking why a back up knot on, belay, assending or any time you put some mechanical device in between you and the rope...wait are the knots at the botom of the rope not called "back ups"?
If in the past these knots have save but one life, wouldn't it be safe to say that in the future they would do the same?
So how long does it take to tie a couple of knots? 2, 3 minutes? Assuming that you didn't "use" them how hard will they be to untie?
Life is good...keep using back ups...make life Gooder!