the rationale has always been, in my experience, that the presidential campaign is the only time that a near majority of citizens give any thought to the political process, at least for a week or two...since there is that focus on politics, nationally, the opportunity to wedge a different perspective into that process could help those fabled grass, local, community roots to sprout...
also, most groups that run a presidential candidate that way never, unless using very high quality drugs, expect to win the white house...their idea is to make it an ad campaign for whatever their "different" politics are, and reach more people at one time than they could ever do by just focusing on an either/or, national or local campaign...
ideally, it's both, and more....practically, we are far from ready...but that's what makes american great..??...
fs