This could open up interesting possibilities like rendering some layers from a child tile and some from a parent tile, and maybe even cross-fade the two. We could then render most features from a tile without waiting for text placement. Together with streaming downloads we could potentially start rendering layers before the entire tile is downloaded.

We could also do compositing operations on data from different sources. For example, a satellite layer that is desaturated everywhere except parks.

I hackishly implemented this way back, and it was slightly slower (40 vs 42fps or something).