Did you see this recent article from NPR? It uses MRIs to examine three conditions in the brain activity of three to five years olds with stories: audio only; the illustrated pages of a storybook with an audio voiceover; and an animated cartoon. And guess what the findings were? The children who had the animated cartoon version had the lowest comprehension (too hot!). The children who had the audio only activated the language networks of their brain but had limited connectivity in other portions of the brain (too cold!) But the children who had the illustrations and the voiceover were just right: researchers saw "increased connectivity between — and among — all the networks they were looking at: visual perception, imagery, default mode and language."
So bring on those stories with illustrations!