I suspect that one trick will work on this keyboard - I mean, when you play the same note in two octaves simultaneously (besides other notes of the chord) in the accompaniment area. This was the doubled note will be the bass note.

I can speculate about what Yamaha thinks: they make it for the customer, who came to the store and thinking which keyboard to chose. Let's say that the customer is a woman, who used to take piano lessons in her school years and now she thinks she'd like to have an instrument at home. She knows that there's so-called 'auto-accompaniment' tool, so she's excited about it. But she looks at PSR-S950 and it scares her to death - how few keys there're and how many buttons! So she sees DGX and she fells in love with it. You know, after it gets to her home, she will lay her fingers on it once or twice, and perhaps a few more times when her friends come by. Okay, maybe she'll play it once or twice a month. But she won't have time to think about the bass note, she'll probably won't be using accompaniment at all. So - this is the thing for those, who want to have an accompaniment on paper. Don't forget - there's only two variations of the style (no 'a','b','c','d' -buttons) and both of them are placed on one button.