Warfare (like most similar combat abilities in that category) adds a 5% total damage increase.
Example:
You have a 'base damage' of 100 (%). 1 point of warfare increases this to 105(%), by taking 100 x 1.05.
If you had 200 (%) damage by having +20 in your chosen main stat (str, fin, int), warfare would add 200 x 1.05, which results in 210(%) damage, which is a net gain over having another point in any other damage stat (like dual wielding). In other words, the moment you have single point invested in any damage type, warfare scales better.
Scoundrel scales the same as warfare if you already have 10 in warfare (+50% damage) and 100% crit chance, but before that, warfare is always better.
In other words, you should always prioritize investing everything into warfare if you are a physical damage dealer and don't need a crucial splash point to gain a new skill.
