No, not really.

True, you can bodge a PC UART's parity to access the 9th bit on a PC, or you can use Mode 3 to bodge a parity bit on an 8051 - but that is not its true purpose!

http://www.8052.com/forum/read.phtml?id=24450

The purpose of the 9th bit is in Multiprocessor Master-Slave communications:
The Master sets bit 9 to indicate an Address byte.
Derivatives with the Enhanced Serial Interface can recognise this & do the address match in hardware with no software loading at all.

See Intel Application Note AP-410, "Enhanced Serial Port on the 83C51FA"
http://www.intel.com/design/mc...270490.htm,

It was discussed at length on the Keil forum: http://www.keil.com/discuss/docs/thread2882.htm

See also
http://www.8052.com/forum/read.phtml?id=60664
http://www.keil.com/discuss/docs/thread2849.htm