Scratch damage is a game mechanic whereby any successful attack always does some minimal amount of damage. This is often used in subtractive combat systems, where the defense is subtracted directly from the damage done by an attacker. Therefore, the target will always take some minimal damage.
The downside of such a system, for me at least, is that it's a hack. It takes a simple formula like
Damage = Attack - Defense and turns it into a (slightly) more complex one:
Damage = max(Attack - Defense, 1).
I also feel that it detracts from a player's skill in developing their character/etc. No matter how many defense bonuses they get, every attack will do some small damage. So why get your defense so high, if it won't mean anything?
Furthermore, this now encourages the use of larger numbers for Hp and damage, so that the scratch damage is truly negligable. After all, if the minimum damage is 1, and you only have 10 Hp, that's still 10% of your health. Even with 20 Hp, that's 5%. And I would rather avoid using larger numbers like that unless it's absolutely necessary.
However, there is one very important upside of scratch damage: it solves the deadlock problem.
Deadlock happens when neither side is able to do damage to the other. If you invest all of your resources into defense, and few into attack, then your character may not take damage, but they won't be able to deal very much either. Thus, you could come upon an encounter where neither side will be able to inflict damage, so battle continues forever. This is especially if you don't have random mechanics like critical hits (which I also hate).
At least with scratch damage, someone will eventually win. It may only be the one with the most Hp or highest number of attacks, but the battle will end.
So I like having a combat system where there will always be an outcome. But I don't like having scratch damage. What are my alternatives?
Alternatives that don't involve rolling random numbers; I want combat to be 100% deterministic. If the same battle is fought, the exact same outcome must happen.
If you want specifics on the gameplay, think in terms of turn-based combat, where battle can be automated (you design your forces, then pit them against others).