Dave,
I checked and really in this case row movement is not necessary, however I do not agree that it means enable movement between partitions. It rather means enable rowid change.
Row movement is for sure necessary when you do segment shring in 10g or when you do flashback table and that has nothing to deal with the partitioning. That's why, since move tablespace leads to rowid changes, I presumed that enable row movement could be necessary

Best
Boris