They could also make the practicalities such as getting a rear case for the phone difficult. Also what happens if you get a grain of sand or grit in between the rear and the module, your inevitably going to end up with scratches on the rear glass housing. That was previously one of the strengths of the Moto X line, they felt like they were built from materials to improve grip, texture, and ergonomics. Everything the Z eschews.
Then we look at the likes of the modules and how useful they would be versus their cost and the 'cost' to the overall design of the Z. The 2015 X was a $399 device with arguably some of the best if not best speakers on a smartphone. Stereo front facing and incredibly loud for a phone, easily loud enough for a room, listening to music. The Z, well we've dumped those and given you a single firing mono speaker which is quieter, but it's OK because you can buy a JBL speaker module for $100-150 for the already price hiked $600-700 Z device, that makes it loud however the speakers are now on the back, it makes the device thicker than any previous X range and of course is going to be damb awkward should someone actually ring your phone whilst your listening to music and you have to answer the phone and hold phone plus bulky module up to your ear.
For every good idea there is a very tangible impracticality and expense not only financially but concession wise to the device design and end users experience.
I in all honesty can not see the Z range taking off and foresee the same fortune as the LG G5 if not a worse one. As bad as the G5 was, it didn't have a $350 projector case !!!