It may not be a common thing, but that doesn’t necessarily make it bad. In your eyes, sure. In mine, it’s a good thing. Build diversity has been championed on this forum for a long time by the devs. Why should we make such a drastic change to a Transmuter when there are other solutions that can be made?
Was Dual Damage a gimmick when you posted your own DD builds on the forum? Such as your [1.1.7.0] King Elessar Pierce/Bleeding Blade Arc Blademaster build? I thought it was pretty legit at the time when you posted it… 
I even started a SSF character based on it at the time. 
Some Dual Damage types are better than others, like Acid/Vitality, Physical/Bleeding, Piercing/Bleeding and Vitality/Bleeding. Vitality/Chaos is one of the better combos as was stated earlier by the OP:
Exactly this. What other items with dual -RR% besides Black Flame Set that are prominently used support the Vitality/Chaos combo? 
- Voidheart (ring)
- Fang of Ch’thon (dagger)
- Symbol of Solael (component)
- Voidsoul Bulwark (shield)
And there are lots of items that have both damage types as well as prefixes and suffixes. Not to forget that both Dying God and Abomination exist too!
Black Flame appears to have been designed to change AAR to Vitality/Chaos when combined with Tainted Power - I don’t see any other way to look at it honestly. The only skill conversion modifier the set has is to change Fire to Chaos on AAR, Tainted Power makes up the difference with only 1 skill point. Sure, you can use other items to get to your intended destination, but if you take a look at the -RR% proc I think you can see it yourself it supports both sides of the coin:
In addition, Black Flame makes SoC to be Vitality/Chaos all on it’s own.
As I said previously, I think the Transmuter needs a bump in damage with a TDM modifier as well as some bumps in itemization damage, but the conversions should stay. Maybe Z has some other interesting ideas on how to improve AAR & FoI instead of changing the transmuters.
I hope things will be reverted and other changes made, but we will see!