On the face of it, the idea of not having SIM slots and messing around with bits of easily-lost plastic sounds alluring. However, a fixed SIM device means Apple controls of your cellular options. Consumers in the USA, who travel internationally less frequently than we do in Europe (for instance) are probably not going to get hot under the collar about this move and could well view it as a big advantage.
Most frequent international travellers could be seriously disadvantaged if removable SIM cards are abolished by Apple, so we need to keep a close eye on developments. In an ideal world, if this internal circuitry does succeed, we would have software settings to change carriers. Anything less would be a disaster for customer choice.