Advantage is significantly less than 1 attack every turn... but more than 1 attack every other combat,
I know but it is 1 attack to end it for the entire fight and it can't be brought back into the fight with healing word.
If there is a PC that is downed or nearly downed and I can finish him off I will attack him instead, but that is rare.
Usually attacking the familiar is the best and quickest way to improve your odds with 1 attack.
I wasnt saying you didnt make the characters sigh
Then what are you saying? You clearly think I am not being truthful about something.
unlesss they are launching a pc caliber attack or yes healing spells which are even more useful when used
They are enhancing a PC attack specifically if you are using them for this and not in an insignificant way.
You are spending telling me elsewhere this is more powerful than a battlemaster maneuver. You can't have it both ways, either it is powerful or it isn't powerful.
and which simultaneously undo the effect of an enemy attack (often entirely) AND also keep the other PC doing their offence ..
See this is the problem. An enemy attack usually does NOTHING to a PC, there is no effect. You say the "effect of an enemy attack". One attack, at any level higher than 3, usually has no effect by itself at all. Attacking the familiar actually makes the attack have an immediate and irrevocable affect.
Unless you can down an opponent, all an attack does is drain the hit point pool, there is usually no effect othert than that. Sure if you can attack the Wizard (not always easy), it MIGHT cause him to lose concentration. Even if you can down a PC, unless you actually kill them about half the time they will still get their action on their turn.
On the other hand killing a familiar kills a familiar dead. It puts a stop to its actions in that fight. Then you concentrate on the next weakest ally until his is dead, then the next weakest .......
From a statical point of view, if a balanced and fully healed party is fighting a hard or more difficult encounter at level 5+ you will be hard pressed to come up with an enemy that attacks for damage where using your first attack on a familiar if easily doable, does not give you the best chance of winning the encounter. This assumes the familiar is being used in combat.
The other thing you should do is concentrate on PCs until they are dead. If you have 3 attacks and you down a PC on the first attack, statistically you should attack that PC twice more. Then the guy standing next to you should attack him to get that third failed death save before he moves on to the fully healed guy.
That may be mean but it gives you the best chance of winning the fight.