Personally, I've always found that mass combat is more of a narrative device than a gaming one, especially in a setting focused on an adventuring group. I've always found that PCs should have an influence on the combat, but the end result should be decided by the GM.
Example, in a recent campaign that I was a PC, after a successful rebellion, my group chose to use our downtime to fortify our kingdom. My character chose to construct a fort within the pass that I thought was a probable route of attack, and other characters did other things to assist the infrastructure. After a year or so, our kingdom came under attack while we were on a quest, and it fell. Our improvements delayed the inevitable conquest so that it was not the mass slaughter that it would have been, and many of our relationships and benefits were able to salvaged and even maintained a guerrila resistance. Narratively however, the GM had moved the campaign to further territories abroad, and he did not want us to be tied to the small kingdom, so the defeat was more a narrative device than a gaming one.
Your mileage may vary, however. A more tactical/strategic campaign will no doubt thrive on mass combat, whereas the luck of warfare will throw a narrative-based campaign into chaos.