It would be more convenient. Given that this is supposedly a security feature - no. Nope. Not at all: what is the state of the data line(s), and how do you know that it matches whatever indicator the cable uses? (There's no way to tell, and you don't - if software is involved, securing stuff becomes HARD, and proving security even harder.)
OTOH, a USB Y-cable physically has no data lines connected on the extra USB-A male connector; it thus provides a far stronger guarantee that there is no data travelling across them. (Not foolproof, but safer than a software-controlled switch.)
That's my point. If I can't tell whether this charging cable is secretly an USB keyboard, isn't it better to explicitly tell my cellphone to treat it as charge-only instead of relying on physical solutions?
My use case is I don't necessarily trust one end of the connection to behave, so I want the ability to just give/take power and involve exactly 0 software. I understand that's not the usual consumer story, and the best argument I can come up with for the general case is it could make any USB compatible wall plug safe to use to charge up.
I'm talking about using the phone's software to block the data connection.
If you don't trust that cellphone won't transfer your private data via USB, I don't understand why you trust it not to do it via any wireless connection.
Android already allows you to only use an USB connection for charging. Setting this as the default when you are in an unknown place sounds reasonable.