- Frost, Arcane and Fire
- Demonology, Destruction and Affliction
- Shadow, Discipline and Holy
- Subtlety and Assasination
- Balance and Restoration
- Elemental and Restoration
And yes, rogues would have competition, but given that daggers can be used by all spell dps and most healers, there'll MUCH more daggers dropping, so in the long run, rogues can expect faster access to daggers, or a lower chance of being unlucky with drops.
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The first case of "imbue insecurity" was, I think, end of wotlk, where people ran FB/FT in battlegrounds, for the ignored armor attribute. Windfury didn't do all that much damage back then (probably the reason they added the 3rd extra strike in cata). It wasn't imbue switching though.
In 4.0, blizz introduced UE and changed RB's effect to a threat/absorb one. This is at best the beginning of imbue switching. HOWEVER, enh would be the one getting the most out of it, in theory, and here we have multiple problems:
-There's a small lockout time after switching weapons
-There's a gcd for changing the imbue, used twice for imbue switching, which would add a pointless 3 seconds/15 seconds, so basically 20% of the time, shamans would be gcd locked by imbues, if they wanted to optimize imbues as much as possible imbue/unleash-wise.
Most people I think decided on an imbue beforehand, and stuck to that. FB became an increasingly punishing choice in comparison to FT, and WF was needed for MSW. RB was not an option, as UF did not exist yet and the imbue being useless.
The first change that would've made imbue dancing actually worth it considering utility would be the introduction of UF, which did not happen before 5.0 (mop pre patch). So there's been basically 1 x-pack where people could use the rb absorb, or fb sprint. And for a very short time, FB was seen as a viable choice. However, even with the big loss in damage due to not taking FT, people preferred to stick to FB because it was just annoying to juggle for that little extra.
I guess the theory, the idea has been around. But it hasn't been a thing. People used UF:RB switches sometimes when in need, I guess. But it would've been to situational to call it being a trend, or a thing, or much utilized.