The problem with having the DB and Morag Tong, is that you won't able to join both. It seems Morrowind and Oblivion sent you down the path that you would be able to experince everything, and not being able to join both would go against that.
Actually, Morrowind didn't allow you to experience everything, especially when you factor in quests that'll cause you to
- Kill every head in the faction
- Greatly displeasing a faction, that they'll flat out refuse you
- Your skills do not match what they want'
- 1 great house per character, unless you exploit the Hlaalu bug
It was OB that broke the mold of not being able to join every organization.
On topic, the Morag Tong is dead, the dunmer are nothing but refugees, and their entire culture is dead. The MT are also illegal everywhere, but Morrowind. However, Morrowind isn't the dunmer's anymore, and the DB has a helluva lot more experience operating underground, where as the MT don't. Not to mention, the DB would be more than happy to eliminate MT members as they cross the Skyrim border, as the DB does not like competition in the least bit.
With the DB, however, more cold business attitude with dark humor and less death cult.