Is Thrash worth using in single target fights? I usually use it on the free proc or I think it affects the conduit “Taste for Blood”?
Not yet, or at least not for most setups I think.
With the right amount of mastery and with Circle it comes out ahead on a full duration Thrash.
TfB conduit also helps its viability.
What I don’t know is what the exact math points are, except for the feeling that it will become much more of a thing next tier with higher stats and conduit bonuses.
I will alternate a Thrash in to proc BT depending on availability of Brutal Slash and before convoke for the extra bite damage from the conduit.
Definitely not something I personally try to maintain on a long fight.
Only exceptions I can think of is something like Shriekwing as the boss transitions I will load up every bleed I can to carry over damage while we are moving away from the boss. Gotta get in those extra few seconds 
I use it when running both Bloodtalons and Taste for Blood and get better numbers than when I don’t use it. With Bloodtalons I frequently end up in a situation where I need a third builder to turn on BT and if I have a clear casting proc I definitely don’t want to use that on Brutal Slash. So I use Thrash instead to get the BT and to buff TfB.