Is it unbalanced to use the same stat to determine a character's ability to attack as well as defend himself? Or should both be separate values?
Both the combat system in Tough Justice and the optional combat stuff in Courtesans use a simple same stat roll off.
Mind you they are propbably the second most abstract combat systems ever written (the first being Gnomemurdered obviously)
It depends on what your game is about, and how much you want to emphasise combat and/or "build design" ability. A game with one base for anything combat-related will emphasise the differences between various levels of it more than games with multiple bases.
In a system that I am working on, "Combat" has one base (Called, appropriately, "Combat"), but it can be tweaked with traits, but this is because I want to be able to say "What do you want your character to be good at doing? Bam, put your highest resource in there. Job done".
Only if defense can be performed by running away.
The number one thing that protects you in a fight is a credible offense. If you are afraid of suffering a heavy counter from my sword, you will be reserved attacking me. If I'm firing an assault rifle at you, you will be hard pressed to take careful aim at me.
By extension, if I know you can not hurt me no matter how you try, I can put everything into my attack. Imagine a boxer with 16 ounce gloves fighting a boxer of equal skill with brass knuckles. Imaging the boldness of the brass knuckle guy.
I think any system that pretends someone can be great at avoiding injury in a fist or sword fight without being good at attacking is full of shit.
Depends.
Let me explain: Balance is only an issue, if you buy several stats with the same ressource. If you roll stats in order, it's totally balanced. There is no place for optimisation.
Only if two stats compete for the same ressource, you need to look what the stats do at all.
So how would a player get this one stat or those two stats, respectively?
Whether it is unbalanced to derive both offense and defense from a single stat depends on the relative utility of other stats. If a system is combat-oriented, the other stats are going to have to do a lot in their own right if any player is going to consider prioritizing them over your combat stat.
So the two questions are:
- What is the relative worth of combat excellence compared to alternatives?
- How does the coverage of other stats compare to the combat stat?
Combat would be one of many stats whose values are decided through point allocation, derived from a single pool. I don't care for random generation.