When you incrementally update a partitioned model, any new categories that are added from data increments automatically get added to the partitions for their partition parents. As a result, cube access time from PowerPlay may degrade as the cube grows. In addition, the model file can become fragmented.
To remedy this, you should periodically rebuild your incrementally updated cubes from scratch (using all data from all increments), at which time you can repartition the model. This allows you to integrate new categories into your partitioning scheme.