I use moi models quite a bit in revit.
Most of the time if i'm taking a model into revit it is intended to be used as a starting point or conceptual mass to build actual revit walls, floors, roofs or curtain pannels from.
The reason is mainly because revit's conceptual modeling tools are really limited and currently not user friendly. It seems to be nurbs based as Michael noted but i consider it more of a 'solids' modeler.
Most designers tend to block out their forms quickly and easily in moi or rhino then export to revit as an .sat where you can quickly convert your surfaces into a revit family type. You can also use moi to create entourage and custom revit families which will otherwise be a nightmare to build in revit like furniture light fixtures etc(asuming you want good looking families default revit families usually look crappy from a visalization standpoint). Nurbs models work and look best in revit thus are prefered over mesh based models.