It's funny, but one thing I always liked about kendo is how much of the bullshit surrounding "fighting with swords" can be boiled down to a minimal set of rules, governed by mathematical trade-offs.
And how once you realize that, almost everything you intuitively know about such a thing is mostly backwards to what the actual strategy should be.
Let's take a simple scenario. I attack you. You defend and parry. Simple, right?
Yeah, not really.
There's basically three considerations that go into that transaction:
1) Energy Costs. Swords are really heavy, especially 2-handers like a katana. Human stamina and muscle endurance are not unlimited. While the energy costs for swinging a sword and attacking are high, defending is not free. You still have to expend a significant amount of energy to move your weapon and repel my attack. Advantage goes to the defender, but only slightly.
2) Opportunity Costs. While the energy cost of attacking may be higher than defending, the attack at least had a non-zero chance of killing you. By definition, your defensive move cannot win the fight, so there is an intrinsic cost in missed opportunity for you this round. Advantage goes entirely to the attacker.
3) Strategy Costs. If you're up against a skilled opponent, they have constructed their attack not just to possibly win, but also to force you into a less strategically strong position should you choose to defend. By blocking my attack, you have allowed me, in a small but non-zero way, to dictate the strategic balance of the following round. Advantage here also goes to the attacker, with how much dictated by relative skill.
When you put these all together, the lost opportunity and strategic costs tend to outweigh whatever you saved in energy costs, and the attacker has "won" the move.
The result of this simple calculus is that there are no purely defensive moves in kendo. None. You're taught that if you ever move in a purely defensive manner, whether to block or otherwise, you have already begun to lose. Attack relentlessly and aggressively, and if you ever find yourself having to block or dodge an attack, use a very minimal set of prescribed motions to always turn the move into an attack of its own.
It's counter-intuitive, and doesn't make for terribly "cinematic" fights, but it's typically the strongest strategy overall.