I think Frontier can walk and chew gum at the same time. In any case, the ships aren't just side benefits of membership. On the contrary, the whole reason people grind rank at all is almost entirely because they want to try out a certain ship. If we ever had to make a choice between the Fed and Imp offerings, then the relative power gaps between them would become a large part of people's choice of allegiance. If one side has significantly greater incentives, then that side will get more folks joining it, hence unbalancing any competitive aspects.
I think if Frontier ever wanted to bifurcate the ranks to make them exclusive to the others, then they would need to reconcile these existing imbalances, otherwise they would create new larger scale imbalances on the galaxy map.