The real problem here is that USB does not define a standard interface for an RS-232 adapter. Proprietary drivers should never have been required for these. Same deal for Centronics printer adapters.
It does, it's called CDC and there are plenty of other adapters that use it. Which is why I think the popularity of FTDI adapters is surprising in some ways.
The problem is that on Windows you still need .inf file with correct VID/PID (this is the case for eg. MCP2200). So for typical user there is not much of an difference between CDC and proprietary driver.
