Yes that is very true, but yet we list plant height, bloom size, foliage type, even bloom time, etc.
Now we all know that those traits also depend on all the normal variances. That same argument is used against listing rust resistance, but finally it made the data base. I know just by my own garden, rust resistant is not accurate for all plants, but it is a good guide (I do feel some plants are overall more resistant to rust). If different strains of rust end up showing more divergence that might make that info less reliable, but I think useful for now. I would be fine with listing "scape strength", or what ever term is derived for stout scapes, as found in full sun conditions relative to other plants in the same garden. That would not cover all the different circumstances (none of the traits do) but it would be very useful I feel. The same for non fading blooms.
Thought this was interesting:
http://www.daylilies.org/AHSfa...
What is the right daylily for my garden, and there first item was color fastness!