Since the z-transform of the zero order hold is 1, why bother including it in analysis or simulation?
The ZOH TF above is a link between continuous and discrete domains in hybrid systems. This is the most convenient mechanism for representing a hybrid system in transfer function form. There is not, of course, a one-to-one relationship between \$s\$ and \$z\$ domains, hence it's a mathematical convenience. In the relationship above, the exponential term should be negative (not positive as given), giving a \$z\$-equivalent of \$1-\exp(-sT)\$ as \$(z -1)/z\$ to be included with purely discrete blocks (filters, etc) and the \$1/s\$ part of the ZOH should be included with the other continuous \$s\$-blocks. The \$s\$-TF of the continuous elements is then transformed into the \$z\$-domain, giving an overall \$z\$-TF.