Someone else correct me if I am wrong, but I believe that is false. iMessage in iCloud is more or less moving Messages from being stored on your phone to being stored in iCloud, as well as a streamlined way to sync messages across devices.
For example, currently when I open my MacBook Pro, I usually get a few old iMessages that come through that I already read/replied to elsewhere (i.e. on my iPhone). iMessage in iCloud will fix this.
Also there is the benefit of less storage space taken up on your device, as Messages will be saved in iCloud instead of on-device. You will also not lose your old iMessages when switching phones or getting a new phone.