Not to dissagree with Kevin because we have had these spine conversation on other boards (Redheads Kevin LOL) and it is all a matter of personal preference. But with that said, to answer "why not'. I use to spine my rods but no longer do unless we are talking a REALLY heavy rod like a 12wt and up. The main reason why I don't is because 95% of all rod blanks today do have a slight curve towards the tip. Dosen't matter if it is a $25 ebay blank or a $350 Sage, chances are it has a very slight curve in it in the last 5" of the tip. Every great once in awhile you will find one dead straight, but that's rare. Just because of the process in rolling them and the thinness of the graphite it is almost impossible to keep it dead on straight.

So two things about that...

#1- when you build on the straightess axis then you can turn the slight bend upward, and use the weight of the last guide&tip top to make the slight curve upward end up straight.

#2- "if" the slight spine of a rod would in some way affect the accuracy of a cast (which means the spine would have to be so powerful that it would change the direction of the weight of 30ft of fly line which I just don't see happening myself, but "if" it could do that...then you would think a slight curve in the end of the tip would have an even more drastic affect on casting, right? So that actually makes the argument for building on the straightest axis in it's own way I would think.

But again like I said, none of this is anything that can really be tested in any way and is all just personal opinions so neither the spine nor straightest axis crowd either one can say 100% which is proven best. I say just read up on it, do a few rods spined, a few on the straightest axis, cast them, sight down them, and see which side of the fence you end up falling on.

Steve