When I used to play, I mainly focused on the rules to help me structure my army correctly and the way I wanted it. The fluff often served secondary, as a source of inspiration to the armies I could create, and the styles of play I might utilise. Hence why most of my marine armies are likely similarly composed, yet due to unique rules have different playstyles.
GW/BL offering a rules only edition? Seems reasonable, and allows people to carry around just the necessities for Tournies or LGS pick up games.
From a marketing perspective, the mystery of why a certain faction works the way it does could lead to the player purchasing more lore focused books from BL, and fallig down the proverbial rabbit hole of backstory.
Is it something i'd say should be standard practice? No, but the option should be their so that seasoned players can grab the latest rules without having to throw out their favourite editions of a races/factions lore.
Looking at Cammerz response, I think some of my thoughts are similar on the subject.
