Switch utilization in dc-dc converters
Erickson refers to this as indirect power.  See page 5 of ECEN5807 Lecture 3.

Given on that slide: $$P_{indirect}=(DV_g)(D'I_L)$$ 

$$I_L*D'=I_g$$
$$P_{indirect}=D*V_g*I_g$$
$$P_{direct}=P_{in}=P_{out}=V_g*I_g$$
$$P_{direct}/P_{indirect}=1/D$$

This is close to what Mohan found, but I'm getting the inverse.  Doing a similar process for a boost I get the same inverted result (1/D').

I hope this helps. 

EDIT: It makes more sense for the switch utilization to be \$P_T/P_O\$ because the power "through" the transistor must always be less then the output power (keeping the ratio below 1.0).  I think this is a typo in the book.

If this is true the equations for Buck:

$$P_T/P_O = D $$

For Boost: 
$$P_T/P_O = D'= (1-D)$$

For Buck Boost: 

$$ P_T/P_O = D*D'$$

The values are different in Table 6.1 of Erickson because (I believe) he's using the RMS values of the switched Voltage/Current so these values can be related to switch loss.  

This means, for example, that conduction losses of the transistor in a buck converter are related to \$\sqrt{D}\$.