There's more than one reason why suing a client for unpaid legal fees is usually considered inadvisable, even if you're the world's largest law firm. Here's one -- your ex-client may countersue, accuse you of a “sweeping practice of overbilling,” and offer your own emails as evidence. And then, insult to injury, a global newspaper like the New York Times may put the story on its front page, including emails like:
“Now Vince has random people working full time on random research projects in standard ‘churn that bill, baby!’ mode. That bill shall know no limits.”
Update: Reuters reports that DLA Piper now has said in an internal memo that the series of emails written by former (emphasis added) DLA Piper lawyers about the law firm's billing practices had been "an unfortunate attempt at humor by three former (emphasis added) lawyers of the firm."