This is an interesting one. I was asked about this when I was with a client recently and why they had been told not to assign a licence to a user that is being migrated to O365 in a Hybrid environment. First of all you have to remember that the primary difference is the integration with Exchange on-premises through a Hybrid configuration. Remeber the Hybrid environment and integration basically just extends your on-premises Exchange environment to Exchange Online, allowing Exchange Online to understand and integrate with on-premises environment.
Once you have established the Exchange Hybrid configuration, you can assign Exchange Online licenses to as many synchronized accounts that are listed in Exchange Online as you want.
The reason why you would do this is that it’s one less step you have to worry about later. When you assign a license to a synchronized account (Using Dirsync or Azure AD Connect) is only an allocation until the mailbox is actually migrated. The license doesn’t create a second mailbox, in Exchange Online, while those mailboxes are located in Exchange on-premise as long as the Hybrid config is complete otherwise license activations will cause those mailboxes to appear.
Hope this makes it a little clearer.