My guess would be that the imbalance might cause slow down/speed up at some crankshaft positions. It's been a while since physics classes, I have a feeling it might happen, but I can't explain the physics behind it.
This is what GM has to say on this topic:
DTC P0315
Circuit Description
The crankshaft position (CKP) system variation learn feature is used to calculate reference period errors caused by slight tolerance variations in the crankshaft, and the CKP sensor. The calculated error allows the powertrain control module (PCM) to accurately compensate for reference period variations. This enhances the ability of the PCM to detect misfire events over a wide range of engine speed and load. The PCM stores the Crankshaft Position System Variation values after a learn procedure has been performed. If the actual crankshaft position variation is not within the Crankshaft Position System Variation compensating values stored in the PCM, DTC P0300 may set. If the CKP system variation values are not stored in the PCM memory, DTC P0315 sets.
The CKP system variation learn procedure is also required when the following service procedures have been performed, regardless of whether or not DTC P0315 is set:
• An engine replacement
• A PCM replacement
• A harmonic balancer replacement
• A crankshaft replacement
• A CKP sensor replacement
• Any engine repairs which disturb the crankshaft to CKP sensor relationship.