If story mode is optional I don't see the problem. I, probably, won't use it, but I don't begrudge people who want to experience the story of Pillars of Eternity but don't enjoy combat, or find combat too difficult even on easy. To address the "they should read books instead comment": (i) an RPG without combat (or with minimal combat) is not the same as a novel, it's a different experience; and (ii) as far as I am aware the Pillars of Eternity story is not available in novel form.
As for the concern that introducing this mode will reduce development time from other aspects, I guess that's true to some extent by definition. That said, given the story mode is presumably meant to very easy it shouldn't take much effort to balance: if in doubt err on the side of easier rather than harder and you won't go far wrong. Of course there are other reasons this mode could detract from other aspects, for example if loot is redistributed in this mode to allow player to get access to hard to obtain items more easily. That said I am not overly worried about this, Obsidian have shown there willingness to push forward release dates to make sure their products are finished so I doubt they'll let this story mode stop them from releasing a good product.