Why doesn't Intel include an Atom core on their CPUs as a next level speed step, like ARMs big.little? It has to be much easier to keep both the high and low power parts the same architecture.
Both Pentium and Celeron are used for standard mainstream dual-core chips and for Atom chips (eg. J4205, Celeron G3900, Pentium G4400)... so in a way they are Intel's trash-can brand.