A human brain upgrade.
Some backgrounds would be fine, others would be distracting because the user might be trying to "fill in the gaps" that the tiles make in the photo or might have a color that's similar to the accent color so that the boundaries of the tiles themselves appear to disappear. These kinds of things make the user work more to discern the information that the tiles are trying to convey in a split second.
The visual impact of background photos and colors can be positive. The information impact of photos and colors can be negative. Microsoft is weighing these against one another. So far, they're sides with being able to convey information quickly, at the cost of glitz.
So at least give us like 5-10 options of made backgrounds like they have in Windows 8.1. Problem solved.