Focusing on the idea of a line of daylilies that, when bred, create nearly identical offspring is a really useful discussion. Like in other breeding arenas, it could produce lines that help us learn about all sorts of attributes, sort of like how we learn from medical science research, and allows breeders to more easily select for characteristics they desire. Just like in dog breeding, highly inbred lines do need out-crossing to regain stability in health and vigor, but the nature of both dominant and recessive genes becomes so better predicted that they actually create formulas and time-tables for outcrossings.
Similarly, a stable plant line also allows hybridizers to easily cull out undesirable traits and increase reliability and performance of desired traits. I know that for many gardeners there is a deep interest in plants that would be very uniform and "true to form." Like Seed mentions, that's clearly not the aim of hybridizers who are seeking to emphasize the new varieties that Cindy notes ... but it would be interesting to hear about more examples that show breeding along lines that are true to form, like Becky identified in Roses in Snow:
Illustrating Admmad's Six Consecutive Generations, with Becky's Roses in Snow lineage, showing how it stabilizes a line and creates lovely variations on a theme:
I know, for example, that Trahlyta seems to be another pretty consistent producer - I can easily spot most its offspring even a few generations down the line.
