Well, isnt the curse seal a result from Orochimaru's experiments with Juugo? So, when exactly did he give her the seal? I'm fairly certain he didnt develope the curse seal technique until after he left the leaf village. That would mean he came back? A likely reason why Anko cant use the transformation (or why we havent seen it) could be she was merely a prototype, another experiment.
But those marks dont neccessarily mean that he wants to use them as containers. Didnt his little 4 man cell elite squad (the ones that stopped an ANBU attack during the chunnin exam finals, came for Sasuke and ultimately got killed by 13&14 year olds) all have a cursed seal?
and i don't remember anyone posting that the mark signifies his container. he used anko as an experiment, kinimaro and sasuke were chosen as his containers for their blood line inheritance. those two got the seal to give them the power they craved
Regarding your question, remember that all Jonins have Genin squads assigned to them in order to receive training. As a famed Jonin, it's part of Orochimaru's job to train Genins as assigned by the Third. Anko simply had the bad luck to be assigned under Orochimaru who was only too eager to find experimental subjects. And much to Orochimaru's surprise, so I speculate, Anko survived the cursed seal, thereby it means she has the potential to be Orochimaru's container. But due to circumstances, namely the Third breaking into Orochimaru's lab, Anko was abandoned. Also, Anko having the earth seal isn't filler. Likewise with Sasuke, Anko probably had a containment seal surrounding the earth seal.
One thing that some people overlooked is that the cursed seal supposedly was supposed to break a person's body down if used for too long, as repeatedly addressed by Orochimaru and the Sound Five. Yet for some reason Sasuke is abusing his seal 2nd without any problems at all. So Anko is "probably" avoiding using the seal to keep her body healthy...I think.
sasuke isn't supposed to stay at level 2 for long periods of time. note that whilst fighting deidara he reverted back to his normal form once he was safe.
Didn't Sasuke just use his Final-Form (Third) against Deidara?
however Anko has the exact same seal as Sasuke which is the Heaven Seal not earth.
and Anko wasnt a container for Orochimaru she's simply an experiment the seal has nothing to do with being a container, however look at it from Orochimaru's point of view. If you needed a new body to transfer ur soul into, woudlnt u want the best most powerful body possible? Orochimaru implents the cursed seal onto the 'containers' that he wants as well as his 'test subjects' simply because he wishes to make use of the cursed seal powers himself once he transfers. However he's NOT going to transfer first and THEN give himself the cursed seal because there's a chance of death remember?
So the smart thing is to brand them first see if they survive, and THEN take 'em over.
It's just like how he gave Sasuke the ability to use CS 2. Why would he risk his precious Uchiha Body dying while goin into CS2? Because he wants the strongest possible thats why.
Look at the guy that he was forced to transfer into, far as I remember that guy did not have a CS mark on him and yet Orochimaru transfered his body in there, and neither did the girl body that he fougth the 3rd with if i remember correctly.
Kimimaro was supposed to be the ultimate body for Orochimaru. The kekkai genkai, the CS2, it was a sweet deal.
As for why Sasuke is able to use the CS 2 so much. Well look at Juugo, he has the CS 2 power and can use it freely and control it freely. My theory on this from what has been said, and the fact that CS has high chance of death, and CS2 has even higher chance of death is that the CS is like a virus that invades ur body and it really depends on your own body strength and will power wether you are able to survive it or not.
Sasuke during the chunnin exam was able to restraint the CS with his will power alone, something that everyone who knew about the CS was surprised to see. Thus this must mean that it plays a huge role in how the CS affects someone. At first Sasuke was in unbearable pain whenever he used the CS.
But it's like anything else u do, if u do it a whole lot u get used to it ur body adjusts. Those that cant adjust to the CS die, those that do, survive into the next stage. juugo was surprised to see Sasuke's control of the CS.
This must mean that Sasuke has pwned the stupid seal into submission in body and mind.
"Pwned the stupid seal into submission in body and mind." I can believe that lol.