Is the US Passport Card beneficial as backup documentation?

It's debatable. From Wikipedia:

The passport card is considered a “List A” document that may be presented by newly hired employees during the employment eligibility verification process to show work authorized status. “List A” documents are those used by employees to prove both identity and work authorization when completing the Form I-9.

and

According to the US consulate in Germany, the passport card can be used as a valid proof of citizenship and proof of identity both inside and outside the United States. However, the acceptance of the passport card as the identity document by private and governmental entities within the USA varies greatly.

So at least one consulate considered it usable with them.

However, it sounds like it might have been intended to be another valid form of government-issued ID, and may well be great for those living near borders, but companies/banks/bars that usually request passports only, may continue to do just that :/

Very valuable it is. As I have Philippine drivers permit, postal I.D. & such. Also a Philippine Visa card. While in America at my bank or other places I find I need use my passport card to prove who I am. This saves having to have my U.S. passport with me in America. My Philippine Visa card does the same there if ask. Citizen U.S. & legal resident of the Philippines. It is a great help in entering America. Just as my Visa is a help returning to the Philippines. Spend more time in the Philippines than in America.

I recently obtained the card in addition to renewing my passport book.

I felt that the online renewal price of $30 was worth it to have a back up Federal ID. When I travel I plan to keep the passport book in the hotel safe and to keep the card on my person as it is proof of citizenship.
I used to keep copies on me and the book in the hotel but then I realized it would just lead an authority/poser to my hotel... (I travelled in Colombia many years ago when it was very likely one would be randomly asked for ID).

I am headed to Cuba now so I think a second form of ID will be a good idea.

For those who frequently take trains or drive to Mexico or Canada, this card is also a good option. Like the traveler above, I also keep scanned copies of my passport readily accessible online, and before I went to Colombia, I also gave a couple of color copies to relatives (there were no cards back then) just so that if I lost the passport I would have multiple resources to access it's facsimile.

I feel the card is just a better and more reliable back up than copies, though I do intend to keep copies as well.

I can waste $30 just walking down the street in Manhattan, so for me, I think this is money well spent, the worst the card can do is nothing, but if needed, it will be worth every penny.