if it’s useful to you, then why not use it?
I think arguments can be made to avoid using something even if it's useful to you. For example, burning fossil fuels is useful for humans, but it will destroy our ability to live on this planet. Of course the pros and cons have to be weighed in every situation. But in regards to the (granted, rhetorical) question about why not to use it, I'm sure valid answers could be given.
What would the manual transmission do? Unless you literally mean it doesn't impact the cars driving and is just there for you to move around. Electric vehicles are not changing gears, so there are no gears to hook up a manual transmission to