I think the case against Sheen is rather flimsy:
1) he showed up for work;
2) he did the shows and they got huge ratings;
3) the drug problems referred to by the lawyers were long-lasting and never a problem for YEARS and no action was ever taken against Sheen; in fact the company made excuses for him;
4) hating your boss and saying stupid angry shit about him is not a contract violation -- if you are willing to show up for work and actually read the shit they give you, you are doing the "essential" parts of the job.
They are threatening to countersue Sheen for breach of contract -- a difficult claim, since they could mitigate their damages simply by putting Sheen on the air, getting enormous ratings, and then in the off season demanding that he get rehab. Hell, they could run shows where he phones in his part from off screen (Farrah Fawcett anyone?).
I would rather have Sheen's case than the studio's case.