Well, I don't think I have ever had a daylily NOT bloom the following year or only bloom so-so; I haven't had daylilies too long though. From a hybridizers stand point, if a daylily only bloomed every other year it would take about 7-8 years (or more depending on zone) for evaluations to take place and well most wouldn't make the cut if this was a trait. Same would go for if they bloomed mediocre because it would take at least 3-4 of the good blooming years (averaged out with the bad years) to get an average bud count. A hybridizer wouldn't get much hybridizing done on these plants if they did either.
I know some say that loading the plant with pods stresses the plant and it may not bloom the following year, but that may be more North. Shoot, James loads all his daylilies full of pods every year and they bloom just fine the following year.
It may have to do with how much watering and feeding they get if they need to recharge a year especially if your natural soil doesn't have much in a way of feeding the plants.
On the rust guestion, it really depends. I have had daylilies that are supposed to be HIGHLY susceptible to rust and they never got it at all (this was before I sprayed or knew to spray for rust).
