Thanks for the insight. Where to start...
The rules are in place to keep things fair for all players and have everyone in mind, not just the poor driver trying to survive a barrage of bullets or rockets. This goes for all rules, not just the spinning one.
In everything you've said, you never mentioned your gunner and his role in your survival. As the driver of a loaded hog, you're responsible for what he does because you're the one moving the vehicle -- he can't race if you aren't racing and he's at your mercy when it comes to going to the navs and getting laps. Spinning wouldn't matter if you didn't have a gunner, but when you do, you're as much a threat as the enemy. You also haven't mentioned those savages shooting at you and their rights in the server. They do have rights, and a desire to survive, and they're also competitive.
While you're spinning away and thinking of the future, what about your gunner and the other players in the server? If you can spin and delay your progress, consequently giving your gunner a better shot, then they too can spin and shoot. Now there's two hogs not going to their navs and just shooting at each other. That's not racing, it's slayer. Once you both start spinning, when do you stop? Both gunners will keep shooting.
You've considered all the spinning advantages and disadvantages, now think about the advantages of progressing: 1) You're getting further away and becoming a smaller target, and 2) you or the enemy may get behind something to block the shooting (like a base, a hill, a corner, etc). Maybe there's more, but those are the two most obvious ones. It does not say in the rules that you have to drive straight and predictably. You can swerve and you can temporarily hide behind something (remember your gunner, if he gets kills while you're hiding, it's more on you than him, at least for the first few seconds).
All these defensive moves (spinning, swerving, hiding) are to give you a chance to survive, but they are exploitable. Things have to be fair for both you and the enemy if the game is going to be worth playing. Your question came up because you want to do whatever it takes to win, but you're not the only one in the server (probably). I'm not saying you have to lay down and die every time someone shoots at you, just keep in mind you're not defenseless when you have a gunner, and that makes you dangerous. The enemy can't stop and shoot at you, and you can't stop or slow to shoot at them.
So Exodus' answer is the best one. It's not a black and white situation, there is no this or that, no either-or. Avoid getting shot, but keep in mind that you're not the only one on your team and the other team has rights too.
Also keep in mind that spinning more than once is just as predictable as driving to a nav point. Spinning is to dodge a rocket you see coming and if there's more than one rocket, a spin followed by going forward and swerving is best. If a chain gun is shooting at you, spinning or swerving won't save you, only getting out of their path will.
BFM_three60 says it wasn't so much your spinning that he noticed, but that for a few seconds you were driving toward the second cliff (nav #3) after getting the score nav. The best thing for you would have been to send him a PM asking about it and get his point of view. As well, when you have a question, keep it short and keep an open mind. You haven't really posted a question, it seems more like defending your actions and arguing the ruling. Nothing wrong with that if you're 100% right, but if you're not right, you won't learn if you're not ready to learn. I was reluctant to write this much because you're not in the right mindset to get anything from this.
Oh Kiwi, you're such a butt inner!