Actually, I started with craft ink and bought all 48 colors! The main reason was embossing. Stamp an image in craft ink, sprinkle clear embossing powder over it and heat with heat tool - instant embossing without having to buy a specially colored embossing powder.
The other major benefit to Craft ink is that it will stick to surfaces Classic Ink won't. The word "Cherish" stamped from the Baroque Motifs set on this piece of designer chipboard is an example. The chipboard was is slick and when I tried it in Classic Ink, the image smudged. Fortuantely, it didn't dry instantly that time so I could just wipe it off. I tried again with Craft Ink and got a very clear image. It was further enhanced it with embossing.
This thickness makes Craft Ink perfect for coloring chipboard. It doesn't get absorbed as much as Classic Ink. I've used Craft Ink on fabric, ribbon, cork, chipboard - both finished and unfinished, and even photographs.
You can use Craft Ink when making coasters. Members of our stamp club remember making coasters a couple of years ago. Craft ink must be sealed but it shows up beautifully on the coasters.
The only drawback I've found besides drying time (and that is seldom an issue) is the thickness. When I'm stamping a particularly small font word or very detailed image, sometimes I prefer Classic Ink.
A great way to get Craft Ink is to buy Stampin' Spots. That is how I got all the 48 colors for a reasonable price. And the small size of the Stampin' Spots is great for directly coloring chipboard.