I don't know if anyone specifically hybridizes for purple based foliage, but it can be quite attractive. Many irises with purple based foliage also show some purple on the flowering stalk. Often, the spathe that covers the flower buds will at least be edged in purple:
Sometimes, the stem itself is purple. It's difficult to capture the color in a picture, but it's usually very striking in person:
Some hybridizers HAVE specifically tried to create irises that are both variegated and purple based. It appears to be a difficult achievement, because only a handful have been introduced.
As Arlyn said, it's useful for figuring out the identity of some varieties. It's also handy when separating clumps in the garden after the flowers are gone.
