There's three things against multi-classing without considering balance:
1. The need for it is far less pressing since the system is far less restrictive than D&D. A big reason to take other classes in D&D is to allow access to new equipment and feats. That's not really an issue here.
2. The game has so many class specific resources that it would clutter the interface and make it hard to read. A monk/cipher alone would have to watch wounds+focus+health+endurance.
3. Since the classes have so many unique mechanics, getting them all to interact in a sensible way would be extremely difficult. Do spells proc carnage? Can fighter/rogues backstab with bladesweep? Does a druid/cipher build focus in beast mode? Do wounds go invisible when a monk/barb rages?
Edited by anameforobsidian, 07 April 2015 - 03:13 PM.