My understanding of it was, if we choose friendship routes, it means you get locked out of all romance and lewd events of the said character (and even some other events can possibly have dialogue differences between friendship route and romance route).
For example, if I romanced Isabel, yet friend-routed all other girls, we will still see all friendship events with other girls, but I guess it can be classified as Isabel-solo route.
Yet, if I wanted to romance Luna and friend-route everyone else, I'll have to start a new save for the Luna-solo route (and this pattern continues for Gracie, and Cordia), hence making it necessary for 4 more extra saves for for each girl's route.
But for me, I think solo-routing a girl will depend on how it influences the ending (I mean if I solo-romanced Luna, yet if the ending ends up not too different from harem [not saying it will, this is only hypothetical situation], then there's no point in me playing a solo route).