Marius is a curious figure in some ways. He may have been a good general (though the biography you provided at least hints that a lot of success was due to others) and he certainly seems to have been a lousy politician, at least in terms of playing the game and forging and breaking alliances at just the right moment. It is true that very often being good at the one thing precludes success in the other; men like Caesar are rare.

That being said, I have some unformulated ideas concerning Marius’ addition of the “head count” to the army. Perhaps the central issue for the populares was the distribution of land to veterans. It’s an issue that had been simmering for more than a generation by Marius’ time, and not a few of those veterans who had once been able to afford the military had been forced down into the head count by manipulations of money-grubbers from the senatorial class. By expanding the ranks with men who had no property or money, but who could now expect to receive a bit of land after their service (perhaps a faint hope considering recent history, but still a hope), he also expanded the support base for popular politicians.

But I think that may be too sophisticated for Marius to have come up with on his own. At least one history of the period that I read, spun Marius’ career to make him sound more like a cat’s-paw for various politicians over his lifetime. Particularly later on, he seems to have been more a tool of men like Saturninus or Cinna, than a powerful politician working with them.

His military reforms were brilliant and necessary. Without them Rome would never have expanded as it did and may even have been forced to rely on mercenaries and/or lost territory, particularly in the east. The addition of the poor to the legions may have destabilized Roman society, but Marius never would have seen the potential consequences nor understood the longer term consequences if he had.