The case in "Marthas and Caitlins" is paper thin, featuring a problem with plane, and is mostly an opportunity to revisit an old character in Colin Sweeney, played by Dylan Baker, who recently finished up a stint on Damages as, you guessed it, a creepy guy. Sweeney, who we haven't seen since the first season, remains a creepy guy who is also indispensable. He likely killed his wife and probably has tendencies along those lines, but he has useful assets, though may be a stretch that he just happened to have the key information for the case. In any case, he proves himself very capable of getting information out of prison thugs for his own release, and then testifying for the case.
Alicia's conflict with David Lee is great stuff, showcasing her place in the firm--both as the low man of the totem pole and as a Caitlin, the one who got in not by merit but by connections. Seeing all of this puts things in perspective for her: no matter how great she thinks she, she's still a lowly third-year associate; and more, she'll always carry the stigma of being in the firm because of Will.
From what I've read, that was Lisa Edelstein's last episode on the show as Celeste. Although she isn't critical to the show, she represents a foil for Alicia, a woman who is different but also feels something for Will. It gives Alicia a great deal to think about, wondering what makes her similar to Caitlin to have the same tastes in men.