Another aspect of rod/reel balance often overlooked is balance for fishing.
When a reel is heavy it forces the tip upward. When rod tip is held high there is potential for a lot of slack and poor hook setting. If you are fishing a strip retrieve (e.g. stripers) you will want to keep the rod tip close to the water. Thus your muscles will have to hold that rod tip down to the water against it's will over a day, weekend or vacation.
The nuisance factor is even more emphasized with two-handed rods because of the extra leverage required to hold the tip down. Since the swing is the majority of the action required to fish, this can be a real pain in the butt and quite tiring too.
Back to casting for a minute -
Ideally I will feel the load in the rod as it increases and releases and not much else. When the reel is heavy it tends to deaden that sensation to the wrist - perhaps because the wrist and arm are having to use muscles to maintain the casting path vertically against gravity as well as front-back.
Proper rod/reel balance just feels good all-around. I wouldn't say it's critical, but once you got it you'll never go back that's for sure!
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