A showcase of how the real world works. At the end of the day, who you know will always trump what you know. Alicia was the better litigator but Cary, by virtue of his huge lead in billable hours and loyalty in ratting out Stern instead of taking the offer, was the better investment for the firm. Or at least he was until Alicia changed the game. She needed the job in order to keep the family afloat, she had leverage that Cary didn't, and she used it. It's not fair, it's not particularly right, and she's going to pay for it in the long run. But it was necessary. Dumping Cary will not be without its costs either. As a junior associate, he has intimate knowledge of every one of Lockhart & Gardner's high profile clients. He's knows the firm's weaknesses and liabilities. More importantly, he knows Alicia cheated and he's not going to forget it. And state's attorney Glenn Childs always has an eye out for people with grudges against the Florricks. Cary's just realized he's swimming in a shark tank, and that changes everything. His biggest liability is that everyone he'd be working for or working against has years of experience fending off sharks, Alicia included.
The most interesting scene for me was the pre-credits scene with the receptionist deciding who to share the big secret with. She goes to Alicia first, only to be blown off. Only then does she go to Cary, who encourages her and complements her even though he has no idea what she knows. Naturally she tells him first. Presumably feeling guilty, she again tries to tell Alicia, only to be blown off again. Alicia is saintly, but she's been upper class for too long. She didn't really think of the receptionist as a person, and that error in judgment (and basic decency) could have been fatal. I've worked in journalism and I've worked in sales/distribution. In both cases, the people lowest on the totem pole are the people you need to try hardest to win over. Receptionists, secretaries, clerks, facilities management, janitors all control access and first impressions. And while the decision makers' actions are largely governed by their self interest, whatever you're trying to accomplish will have zero impact on the lives of the gatekeepers. Any help they provide is a personal favor out of the goodness of their hearts, and you can't ever forget it.