It is also unnecessary 99.9% of the time. Nearly all RS232 devices and hosts will work just fine with TTL voltages (+5V/GND).
I wish that were true, but it sure as hell isn't!
Sure, MANY devices accept TTL voltage RS232, but a big number DO NOT. So if you depend on that USB-RS232 adapter, you'll be left standing around, looking like a moron.
Just had to configure a brand new smart PDU a while ago... Absolutely no activity to my company laptop via USB. Bad cable? Incorrect wiring? Defective PDU? Nope... Plugged that cable to a server with an actual RS232 port, and it works flawlessly.
I wish I could find USB-RS232 adapters that actually output the proper voltages...