Seeing as no-one else has posted here is my offering on why you shouldn't install SQL on a DC.
- SQL is very memory hungry, installing it on a DC will cause contention between SQL and the OS
- SQL is also very disk intensive, same reason as above for why you shouldn't do it.
- SQL is very network intensive, depending on the number of users in the domain and the size/use of the SQL database there will be still more contention between it and the OS.
The list goes on and on, basically, it's a bad idea and to answer your question with a question, knowing the above, why would anyone want to install SQL on a DC? they'd have to be nuts .
Hope this helps a little