The DMTs may be better- I've never compared them directly to the card linked above- but the neat thing is the form factor of the card. I'm not a big fan of diamond sharpeners for the most part. They work phenomenally well for flattening water stones but I generally only use them for a few sharpening applications. The Japanese Atoma plates are much better for sharpening than a DMT and will outlast them by several times. Nothing wrong with a diamond sharpener though for a small kit.

That said, I don't generally see much need to sharpen a knife in the field. Maybe if you only had one knife or tool and had to use it for weeks on end or even months, it might be nice to be able to touch it up. My habit is to have multiple cutting tools and I generally favor knives with steels that will hold an edge longer (eg REX-45, 154CM, HAP40, M390, etc).