While the mode of production might be the base of a political economy, its clearly not true that there is a strictly one way relationship between politics and economics. People's lives, and society at large, are influenced all the time by government policies, and those policies are shaped by the form of government. Even if that form was produced by the economy, it in turn influences the economy that produced it.

If there is some day a giant communist revolution in your country, people are going to have to make some initial decisions about the political structure. Who controls the army/workers militia? Is there a "vanguard party"? Who is in charge of that? How centralized is the distribution of resources? How are the revolutionaries supposed to make those decisions without any political philosophy?