Viable means it works, not the best option, but workable nevertheless.
Dual spec is the better option, but I don't think we should always assume that those starting a priest already have an 80 parked at this point. So without it, Smite is your best option if you wish to level as a healer.
Last time I checked, Smite is only unbearable from 1 - 20. As soon as you get Holy Fire and Glyph of Smite the fun starts. Running instances will also get you better blues, which equals to better damage when soloing.
I'm not discriminating Shadow here, it does do pretty good damage. But people have different styles of play.