As a choice of 0xD000 seemed to be particularly popular (likely because the built-in monitor has a shortcut for executing the program at this location)
AFAIR this was added in the second version of the NAS-SYS monitor, as a service for the parallel delivered new ZEAP editor/assembler (*1,2), not so much for general purpose (*3).
there are many conflicts in software at this address, particularly the NAS-DOS operating system, and a variety of programming languages (e.g. BLS Pascal, the predecessor of Turbo Pascal).
The conflict may not be as real as you may assume. Software isn't all the same, nor is its usage situation.
For one, people who bought a ROM based PASCAL system don't neccessary switch for a different one ur another language on a daily base - if at all. These decisions where usually long term. Nervous flipping between versions and even more so languages is a rather new issue (*4).
Especialy the conflict with DOS isn't one, as people adding Disk drives and DOS in ROM, of course switch for a more flexible, disk based software (*5). After all, why permanent wasting precious address space for a compiler that can as well be loaded into RAM - and removed after it done its task.
Last but not least, next to all ROM application software was also available on cassette (includng BLS Pascal), to be loaded into RAM (*6). With Pascal (or some extended BASIC) installed the situation was no diferent form any other system in the need to load an assembler from cassette to write some subroutine in all comfort (*7,8).
I'm prety sure, that many of the seamingly conflicts will vanish when thinking it from common usage szenarios of back then.
*1 - Conveniant named Z80 Editor-Assembler Package
*2 - The older version was made to work with the original NASBUG.
*3 - Similar
J was reserved to start BASIC residing at
*4 - Remember the computer/manufacturer and language wars? People were comited to their environment way more than today.
*5 - The ROM Pascal, like the BASIC, only supported cassette as storage - not realy useful with disks.
*6 - Thanks to Jules for reminding me about that.
*7 - Then again, instead of going thru all of this, one wrote it by hand and poked it from the HLL program later on.
*8 - Not to mentioned, that one of the claims of Pascal was to generate code good enough to not needing any Assembly modules :))