I think what you are describing is weight and not balance. I agree it's easier to cast something light all day, but as far as that being a function of balance, not so much.
See where that rod actually balances with both reels and I'll bet it is within far less than an inch of each other.
OK, I took the biggest and smallest reel I own and put them on my lightest rod weight wise. My 7 wt. Winston. This rod wieghs less than my older 5 and 6 weight rods. Here are the reels I used. The big one is a Tibor 13/14/15 wt. reel loaded with a Carron Jetstream long belly line. It weighs 1 lb. 11 oz. with the line reeled all the way in. It is 5¼" across and is grossly oversized for this rod. The small one is the tiniest reel I own with a line on it. You get the idea with the two together. It is grossly undersized for the rod.
The line is reeled in all the way, making this as exagerated as possible. Note that with the monster Tibor, it balances on one finger well to the mid grip. People confuse heavy with balance. This reel balances well within where you could use it with your hand on the grip.
The teeny tiny reel, balances off the grip to the forward, but not to the point it would make it hard to use.
In order to get a rod/reel so out of balance that it would be a pain, you need to try really hard to do it. If you are going to make it where it is off in an annoying manner, get the smallest reel you can find. This whole going lighter and lighter is going the wrong way for balance. For weight you are swinging all day then sure, but that is not balance. That is weight. Hence the TWO