They're more expensive (influence wise) and less efficient than Steam boats. Enemy AI attacks them anyway.
I really see no point in using charter routes beyond the free influence ones. Even if you for some reason can only field Schooners, building one yourself is cheaper.

I can see only one scenario where it's worth it. When your entire merchant fleet got rekt all at once, you're focusing your production on military ships and need a merchant fleet ASAP. Way too specific and the enemy will attack them anyway.