Should the nexus be the "top of the line" android?

People go on and on about how great the nexus is, but the specs are usully always a step bellow that of non nexus devices. If you are gonna get a "pure google" phone, shouldn't the phone be top of the line as well? What about choosing what google services you can have on the phone. Example why does apple make iBooks optional, but Google Play books is mandatory?