The real value of the Passport Card is for those who essentially commute across the US/Canada border. There are people who literally make hundreds of crossings a year - who live in the US, and work in Canada or have sales routes in Canada or where the nearest supermarket is in Canada . For those in that kind of situation, carrying and showing a card is much easier than carrying a conventional passport every time they have to cross the border.
In states offering enhanced drivers licenses, the Passport Card is not that necessary. However, in border states that do not offer an enhanced drivers license, the Passport Card is likely pretty convenient.