I would prefer rika only be narrator if they are never gonna use the others once and awhile. I await times when they may be out (perhaps on more alo-hello/love-hellos) that they replace the missing person with someone we miss... whether its rika, erika, yui, nono, etc. I think it would be awesome if the other girls got to show up for some stuff here and there.
I think don't know if that would every happen though, unless they request it at the SNS a lot