War is good in-game (RPGs) when:
1. the characters have a direct stake in the outcome
2. characters can directly affect the outcome
3. the characters can engage in the war at several levels (leadership, combat, espionage, diplomacy, training, etc.)
4. it will dramatically change, for better or worse, the surrounding world
5. the characters will profit (I don't necessarily mean monetarily) if the war turns in their favor, and they will suffer if their cause loses
6. the War is truly dangerous, and potentially lethal to characters, as well as to everyone else involved
7. the war can be fought in an interesting and different way than normal combat
8. the war leads to character change (status in society, rank, privilege, heavier rpeosnsibilty, opportunities for different types of adventure, loss of comrades, any kind of change common to war that men either benefit from or suffer through)
9. the war teaches lessons, of all kinds
10. the war ravages, and also leads to rebuilding
11. Comrades are killed, and/or commit heroic acts
12. new inventions, technologies, and advances are made, and war usually leads to some type of change or advance to both societies and technologies (even if it is only war-technologies, such as weapons)
13. war tests the Strategic genius and capabilities of the players
14. war tests the tactical genius and creativity of the players
15. war leads to bizarre, unusual, or momentous new (or renewed/ancient) discoveries
16. the war posits previously unknown threats and reveals hidden enemies
17. the war leads to infiltration and subversion of things and areas and groups and individuals previously thought safe and reliable
18. the war leads to powerful and inescapable moral dilemmas
19. the war leads to previously unknown/unexamined weaknesses in the characters
20. the war leads to many opportunities for acts of individual Heroism
Not every War has to have all of these characteristics, it will depend upon the nature of the War, and who is involved, and how the characters are involved. But the best Wars have many of these characteristics, including ones I didn't mention.
I often use War in-game, because in-game War is good. Very good.