My understanding is that at the beginning the induction motor is deexcited. There is no magnetic flux in its magnetic circuit. In these circumstances you have these initial conditions: \$I_d(0)=0\$, \$I_q(0)=0\$ and \$\theta(0)=0\$. Then in case some torque is requested it is necessary to build the magnetic flux at first. During this "excitation" period the reference value of the quadrature component of the stator current is \$I^*_q=0\$. As soon as the magnetic flux in the machine achieves its nominal value the machine can produce torque i.e. you can start requesting non-zero reference value of the quadrature component of the stator current.
As far as the magnetizing current usage I would say that it is only for sake of simplicity. In case I write so called "current-speed" model of the induction motor in rotating coordinates coupled with rotor magnetic flux I have following differential equations
\$\frac{d\psi_{2d}}{dt}=L_h\frac{R_2}{L_2}i_{1d}-\frac{R_2}{L_2}\psi_{2d}+(\omega_s-\omega)\psi_{2q}\$
\$\frac{d\psi_{2q}}{dt}=L_h\frac{R_2}{L_2}i_{1q}-\frac{R_2}{L_2}\psi_{2q}-(\omega_s-\omega)\psi_{2d}\$
In case the (d,q) reference frame is coupled with the \$\hat{\psi}_2\$ we can write \$\psi_{2q}=0\$. The \$(\omega_s-\omega)\$ coefficient is the slip speed or rotor speed \$\omega_2\$. In case we take into account the aforementioned conditions the equations simplify in following manner
\$\frac{d\psi_{2d}}{dt}=L_h\frac{R_2}{L_2}i_{1d}-\frac{R_2}{L_2}\psi_{2d}\$
\$\omega_2\psi_{2d}=L_h\frac{R_2}{L_2}i_{1q}\$
We can define rotor magnetizing current in following manner \$i_{m2}=\frac{\psi_{2d}}{L_h}\$ and then we can rewrite the above mentioned equations into following form
\$\frac{di_{m2}}{dt}=\frac{i_{1d}-i_{m2}}{T_2}\$
\$\omega_2=\frac{1}{T_2}\frac{i_{1q}}{i_{m2}}\$
where \$T_2=\frac{L_2}{R_2}\$ is the rotor time constant. The last set of equations is the base from which the Microchip difference equations were derived from.
In case we can calculate the rotor speed \$\omega_2=s\cdot\omega_1\$ and measure the mechanical speed \$\omega_m\$ we can sum them and we obtain synchronous speed which is magnetic flux speed of rotation \$\omega_1=\omega_2+\omega_m\$. Integration of the \$\omega_1\$ gives us the transform angle for the Park and inverse Park transformation.
