My understanding is that a plant patent only prohibits asexual reproduction like cuttings or division.
If I'm right, it's always legal to collect seeds because that is sexual reproduction even when the plant was self-pollinated.
Of course the offspring are not likely to be identical to the parent, but you can select and keep crossing back until you get some individuals that you like. Then you can propagate that vegetatively to your heart's content.
Thus it is useful to know the best propagation method for some patented hybrid. After propagating it by seed for one or more generations, that method ought to be effective for your F2, F3 or whatever offspring.
