2s are defense in depth, when you think you will be a likely target for elimination aka you are likely to lose. In escalating spoils you worry about that when an oppo has now or may have (say by cashing a set) enough force to eliminate you and you are a desireable target for elimination, holding 3 , 4 or 5 spoils,
Concentrating your forces in key locations gives you some defense- how may troops would an oppo need to deploy to be sure to take you if you have say, three or four terts scattered around the map each medium to big stacks, say with 6+ troops o?? none near an oppo tert with 4+ troops on? , but is particularly strong on offense. three or four terts scattered around the map each medium to big stacks, say with 6+ troops on could attack in any number of fronts. Add in you cashing a set at some time, maybe not this turn, but some time, to give you a big deploy n and you could possibly eliminate an oppo, grab a set, sweep the board, yawn, another victory.