ftp/curl is a widely used port. Around 500 ports are using this. So requesting to put it in USES.
Generally, Uses/* macros provide logic beyond a straight dependency. When they do contain only depends, it’s typically for things where the actual dependency may change. For example, Uses/jpeg.mk only adds dependencies, but it’s because the JPEG library we utilize changed (and may well change again).
I could well be wrong here, but I’m not sure that there’s precedent to add USES=curl when the actual dependency lines are so simple.