• The advantage of not keeping shipments in sync is that you can retrieve in bulk the required warehouse locations from Amazon for a batch of items without having to create any shipment plans or Working shipments in Seller Central.
This way you can do a preview of the fulfillment centers where your items would need to get sent. They would get added to Local shipments which are created in the software but are not registered as Working on Amazon.
Local shipments can be deleted at any time since they are not visible in Seller Central as opposed to Working shipments which you would need to cancel or delete. The problem with cancelling Working shipments is that if you do this too many times Amazon may suspend your account.
• The disadvantage of not keeping shipments in sync is that if you want to register some of the Local shipments you might get an error.
This can happen because the shipment preview calls based on which the warehouse locations are assigned to items can expire in a matter of minutes. The more time past since first creating a shipment the more chances the preview call for some of the items in it will expire and so you won't be able to register the shipment.