I 100% agree that rulers should be able to lead at least the Capital legion - I mean, we talk about Alexander the Great's conquests, not his indirect oversight of the conquest of Persia by his Generals - but I'd also like to see a two-tier loyalty system. A few of the -5% Cohort Loyalty Chance things I feel could be revised to give the cohorts a chance to build STATE loyalty rather than CHARACTER loyalty. In this system, a cohort loyal to the State would add to the power base of the ruler, where one loyal to a character would add to his.
You could have a redudancy where a relevant minister - like the guy in charge of generating military experience - commanded the capital levy if the ruler was unable.