I actually haven't seen lots of post against that idea. Especiallly no strong feelings.
Lets make list of the different advantages of adding or seperating:
Adding:
1) Less Battletomes > less rules and bloat
2) Quasi Range expanion for DoK (which is needed)
3) Opens up different playstyles within a battletome by choosing Morathi or Malerion with different allegiance boni
4) easy start for collecting the army as it can be added to a existing DoK force (rather than getting a full blown new army with books, markers and models and what not)
5) reintroduction of a true Dark Elf successor as a complete faction
Seperating:
1) Allows for confontration between Morathi and Malerion, rather than cooperation
2) Umbraneth can have a unique look (like IDK) rather than being forced to tie in with DoK/DarkElf
Feel free to add to the list.