As I said, if they are not the SQL Server admin and responsible for the backups/restores, patching, on-call, etc. there is no reason to give a sharepoint admin elevated access.

Creating new service apps or content databases is not a daily or regular activity and if creating apps and content databases is happening frequently, there seems to be a problem in defining how sharepoint should be setup,how it needs to be configured and how it's used. So even more of a reason to not allow elevated permissions. Permissions needed for installation or migration is different than day to day.
Sharepoint admins doing regular activities via Powershell rather than Central Admin console would be a choice rather than a requirement.
If nothing else and someone was forced to provide elevated permissions, I still wouldn't give a sharepoint admin elevated privileges as it's just not congruent with the principles of least privileges. I would create an account that is normally disabled and only temporarily enable it when absolutely needed during a maintenance or change window, especially if they are adding databases, making SQL Server security changes for something they do not support.

Sue