Ask Mr. Smarty Plants

This USDA Plant Profile Map shows that Lobelia cardinalis (Cardinal flower) grows natively in Washington County, RI. Since you have already grown it for a long time there, that at least partially rules out a problem with incompatible soils causing the variegation. We looked aat all the pictures in our Image Gallery of this plant and some more online and saw no variegation, so what you have may well be unique. Here are propagation instructions from our webpage on Cardinal flower:

"Propagation

Propagation Material: Seeds
Seed Collection: As seeds approach maturity, the capsule opens slightly at its top. Check capsules at both upper and lower portions of the stalk. Store dried, cleaned seed in a sealed, refrigerated container up to three years.
Seed Treatment: This species requires or benefits from a three month period of cold-moist stratification in the refrigerator.
Commercially Avail: yes"

Generally speaking, it is difficult to propagate a hybrid via seed; whatever the variation, it rarely breeds true. The leaves of this plant are long narrow blades, so the variegation might not be worthwhile to other gardeners. Since it is a perennial, you could try taking root cuttings and see if you can repeat the characteristic by vegetative propagation.

Your original question was whether this characteristic was worth propagating. That would be entirely up to you - if you enjoy experimenting with propagation and perhaps creating different leaf markings, that would be worth it. Whether there would be any commercial value, we couldn't possibly say.