Sandhills Bass Anglers, North Carolina

If you set it up properly you will never cast out your old line.  Plus if you only fill your spool up half way you are reducing the full potential of the casting capability of the rod and reel setup.  You can gain (especially on lighter lures like crank baits) tons of distance by going to 10lb line and filling up to spool capacity.  On heavy lures it doesn't matter much how full you fill your spool. 

if your not casting the old line out then i dont understand how it is giving u better casting capability, i dunno, ive never done this and ive always been able to cast far. Seems to me the more crap line u have the better chance of getting a backlash, less line = less chance of insane birds nesting.