I buy seedlings because I may not be able to afford the parents at the time if they are newer ones or I may just not want to buy the parents and take the time to make the seed and plant them when I can go ahead and buy a seedling that has the parentage I want and someone else has done the work for me in making seed, planting, growing. This saves me time in the long run by not having to take a year or more to get the seedling I want to use.
I have seedlings I have bought but will never be registered because I either do not own all of it or the hybridizer I got it from said not to sell it, give it away, or anything else except compost it once I'm done with it. The reason he didn't register it himself is because it was shorter than he wants for his program, everything else about it was great.
I use these for hybridizing because they bring something into my program I want to use.