Minecraft wiki entry for Rabbit_Stew, associated text: If the ingredients of rabbit stew were eaten separately and the mushroom had become half of a mushroom stew, they would restore a total of 16 ( × 8) and 19.2 saturation points. Without counting the mushroom, the separate ingredients would restore 13 ( × 6.5) and 15.6 saturation points. Therefore, crafting rabbit stew results in a net loss of restorative points, though it is quicker than eating all the ingredients separately.
Rabbit stew restores more hunger and total food points (hunger + saturation) than nearly any other single item (the exception is a suspicious stew with Saturation). This is balanced by the point that the stew does not stack, and with its complex recipe it's not so easy to make "on the road". The suspicious stew shares the unstackability, but with only four ingredients it can at least be crafted in the inventory.