Its important to have a Passport card. I for one, applied for the passport card with my passport book. It comes handy as ID and another form of US citizenship proof and it is very handy.
What if, there is an immigration raid at your work? How will you show US citizenzhip? The passport card is the answer
What if laws change in a state ( like Arizona) ? You get stopped by a police who asked for your citizenship? The passport card is the answer
My US citizen friend in Kansas City was stopped by the Police recently. The cop asked him if he has a green card and he doesnt have the passport card. He argued with the police if he needs to bring his passport book when driving around Kansas City. Nothing happened to him. He was stopped because he was driving a corvette and he's not even over speeding, and he didnt get any ticket. An asian guy driving a corvette..hmm..typical racial profiling ! However, if he had the passport card, it would've been less hassle for him
( I dont know if it was legal for the Kansas city cop to ask him for his immigration status )
In any way, it is very beneficial to have two proofs of US citizenship, one ( the book ) is in your home/bank safe, the second (passport card) is safely tucked in your wallet.