Most modern application UI can't really be encoded as vectors, there's lots of raster stuff in there plus bitmaps. Many mobile apps right now are a big pile of

Most modern application UI can't really be encoded as vectors, there's lots of raster stuff in there plus bitmaps. Many mobile apps right now are a big pile of PNGs and/or raster filters (like drop shadows, etc). So you end up actually needing to ship scene graphs along with bitmaps. You can certainly leverage that, but it also increases the resource demands (and thus power demands) on the client.