Advantages of Heterodyne vs Homodyne
The advantages of a hetrodyne TX are basically that the modulator does not need to run at the output frequency, and it is MUCH easier to build a modulator having good linearity at 70MHz or so then at 24GHz.
Because the modulator output frequency is fixed, you can also add useful filtering here to restrict the occupied bandwidth at the output of the modulator, much harder to produce a 2MHz stopband at 24GHz then at 70MHz. 

Now you do see homodyne designs, usually where modulator linearity is unimportant or at lowish frequencies, most of the FM broadcast sets are homodyne, as are the 433MHz SRDs and most of the AM broadcast stuff (Which actually often looks more like a modern switch mode supply then a classical transmitter), the common thread is that none of these things are particularly fast by modern standards. 

One interesting development has been the use of digital upconversion directly from a complex baseband signal, you can think of this as homodyne or even zero IF hetrodyne depending on your view, this is usually seen at VHF or below with some parts that interpolate and work in a higher Nyquest zone getting up into the UHF, however you can view the DAC in such things as being a mixer, so these are hetrodyne if you squint just right.