i'm curious about that too... i got a few dwarf sag thrown in free with my frogbit and the person who sent them did include root tabs. i am wondering if that was a generic freebie sent with any plants or if i need to get some more to replace them once they run out. the plants were growing for a week in qt when i had them in a vase with playsand, no tabs and flourish but of course that's not very long.
in one tank it is soil based so it grows like crazy all over the ground and is only 2-3 inches.
in another tank i have sand and i put root tabs under a few. they dont grow as much and the leaves are thinner but they are green. they also grow slower and have off shoots but they havent covered everything like the soil tank. i have not added more root tabs under the off shoots.