Oooh, a fun question! Why you gotta ask just Marxboy, though? There are other Reds on the board, you know 
Why is it so important to a Marxist worldview? Quite simply put, it’s because Marx himself developed it. Or, rather, he put the materialism in the dialectic.
The dialectic, at least as modern philosophers know it, was developed by GWF Hegel and basically says that contradiction is the motor of change. The existence, Hegel says, of a given concept logically entails the existence of its opposite (the ‘thesis’ and ‘antithesis’). The contradiction and conflict between the two forces change and progress into a new order of things (the ‘synthesis’).
Unfortunately, Hegel saw the dialectic as acting only through ideas, or more specifically, the Spirit of History. History didn’t have a plan or a goal, i.e. it wasn’t an outside, alien force influencing the development of humanity, but nevertheless it was an overarching Idea that made itself felt through historical development.
Marx, on the other hand, argued that the cause of conflict and change was firmly rooted in the real, physical world, in the development of society and the social situations in which people found themselves. There was no Spirit of History or God or even a Categorical Imperative, just men and women living and working within society. And since society, at least since the development of agriculture, has been based on real inequality, the contradictions inherent in such a setup would lead to further conflict and social change.
In short, no. You cannot be a Marxist and an idealist at the same time. Idealists think that merely arguing for change will bring it about as more people accept and understand the idea. Marxists think that change can only come about from physical action.
Long winded, I know. But there’s a lot of stuff to cover.