Pen hardware is usually made to "press in" to a coresponding part on the pen barrel. This press fit usually requires that there be a certain amount if rigidity there to press into. Most of the materials we use for pens is somewhat less than rigid for a number of reasons - soft wood or plastic, or material so thin it cannot hold up to the internal pressure of the press fit. Hence the use of cheap, readily available brass tubes that can be easily glued into the less than rigid pen blank and lend it's rigidity to the process. Is brass necessary? No, but it is cheap and has working properties that are desirable for this use. Would plastic work? Maybe, but I think that any plastic that would be rigid enough with that thin of walls would maybe be more expensive than the brass.
Now, if we were to get away from the idea that parts had to be press fit, and could just be glued in, that may make a difference. But as thin as the material gets on some (many) pens, having a nice sturdy, rigid material glued inside really makes lighter and stronger pens possible.
Does this help? Or am I just blathering on here? [8D]
Scott.