They USED to be attractive clients, then after Jimmy finds out they did, in fact, embezzle the money into cash, he takes a bribe to keep quiet. Now, they are a loose thread. They strong-armed him into being his lawyer when they weren't satisfied with the answer.
The implication seems to be that the money hasn't been spent yet. The legal fees aside (they probably can afford it outside the stolen money), it makes HH&M look great as a firm if they can turn a possible 30 year sentence into 18 months. At least that's how it seems to me. Even if they did spend it, they can implicate Jimmy for their sudden disappearance and for obstruction of justice. Not that it would be something Jimmy couldn't get out of, but it would be a big news story at the very least. Plenty of reasons to not want to be accused of any wrong-doing of his own. It could tarnish his budding reputation and it could also lead to Chuck relapsing into his electromagnetism allergy.
The Ketelmans are great characters, and I think very indicative of the tonal shift this series takes. I really wish people were on board with this show now, because I think it deserves the same Breaking Bad level of adoration. Maybe not to the point where Saul is showing up in insurance commercials, but so Bob Odenkirk can make more movies.