UCA0STE, UCB0STE in SPI Master mode

You do not need to use the STE pin in 4-wire master mode. You can control the slave's STE using a port pin. See figure 16-2 (page 501) of the User's Guide

The STE pin does not have to be used when there is only one master device.  If your system is simple, and there is only one device acting as a master SPI device, the only pins that need to be used by the SPI module are SCLK, SIMO, and SOMI.  In my applications, I usually use the port pin that STE belongs to (usually P3.0) as a general input output pin controlling the SPI's chip select line. 

ste is an INPUT in multi master mode (4 wire).. The normal mode is 3 wire mode and you use a general i/o  (you could use one of hese pins bt you have to configure it as a general i/o) port pin to control the target devices chip enable.

This pins generally have alternate functions, and if you need to maximize the use of the various USCI hardware, you cat use these pins... 4 wire spi mode reduces the options that you have..

SPI is generaly called a 4 wire interface, but TI doesnt count the chip enabe pin as art of the interface.. This is a commn point of confusion so in 4 wire mode the interface is actually 5 pins