But how do you know you will be getting disadvantage next round so you can cast advantage this round?
Let's say it is a situation that is pretty constant like underwater fighting.
I have disadvantage on my attacks this round and next round guaranteed because I forgot my spear.
7th level EK, I can either get 4 attacks with disadvantage over those 2 rounds taking the attack action twice, or cast true strike round one and make an attack as a bonus action at disadvantage still, then next round use attack action getting in two attacks the first normal (from true strike countering disadvantage) and second with disadvantage.
4 disadvantage attacks vs 1 normal and 2 disadvantage attacks = pretty much the same thing, so why cast true strike at all.
Now the 11th level EK in same situation can have 6 attacks at disadvantage or 1 normal and 3 with disadvantage, I would much rather take the 6 attacks every single time.