Well there's like what, 10 hours of story? WIthout companion input, it'd be an awfully short game!
I'm not complaining *exactly*... Just an observation that the story is fairly short. I'd rather a good story that was short, to be clear, instead of a crap story that was loooooong (looking at you, NWN2!). And I do think the main story of DAI is engaging, but it certainly features a good chunk of companion time.
I do actually think romance adds a lot - more like the sugar in your cake, rather than the icing. In the same way that adventure adds a lot. It's all part of the package: fun stories have lovers, villains, heroes, and danger.
It's slightly different with games, particularly bioware, because the companions ARE a central draw. And that's okay, imo. It makes the story content modular, as in you can skip bits or become involved as much as you like to a certain extent. (For example, I cba with Sera, so I skipped her this pt). From a design perspective I am sure that must be useful, and solve all sorts of problems - namely, how to please everyone. Otherwise if you want a really involved story, you need something like Planescape Torment, which has limited mass appeal and can be a tad overwhelming.
It also adds a unique selling point. The interesting characters become little sections of story to explore all on their own, and open up opportunities to learn more about the world. The classic example would probably be ME2, which felt entirely character focused to me, in a good way.
Naturally, people become pretty obsessed with your characters. Being able to tie a romance into those companions makes them more "meaningful" to your protagonist and even more interesting to your players.
I don't think romance alone is selling the game, though. If it were, those Japanese style chat up games would be a heck of a lot more popular than they are. Too much sugar just makes you sick. But there is certainly a strong *awareness* from devs that romance sells... possibly too many resources are devoted to it? It must be a difficult balance to strike.