The key ‘argument’ would be that of forcing key values/secrets across the development environment. if the values are held in git they can be changed at the local and branch levels or even git ignored if a developer wishes.
The term ‘argument’ would mean just that, Doppler can be used as an Ops tool to enforce central settings on the developer environments, which maybe a step to far.
To limit the amount of debate such a setup would cause, I use Doppler at the CI level - developers can use whatever values they want in their local environments and the repos, but the instant a central build takes place it is using values set by Ops.