They do this a lot with nectarines, too, which is trickier for me. I'm not that picky about plums because I like both, but I'm really torn on nectarines. I like it when they're just a little crunchy, and the flesh comes away from the pit easily. The ones that are softer and juicier have a little better flavor, but the flesh sticks to the pit, juice gets everywhere, and striny bits get stuck in my teeth. I knew that nectarines were a peach cultivar because the softer ones that don't come away from the seed pretty much taste exactly like a good, ripe peach to me. I didn't know that there were also two subclasses, though. The ones that come away from the pit, and the ones that cling to the pit. I just figured I liked them a little unripe.
Now that I know they are two distinct types, though, it kind of bothers me that they arn't labeled. I'd totally bag up more of the ones that come away from the seed than the juicy ones, despite the taste difference. I guess you could say it's one of the only times I'd spring for texture over flavor. My beef with peaches is that while they are delicious, they make a huge mess and eating somthing with a fuzzy skin doesn't feel right :P
A nectarine cultivar that was firm, came away from the pit cleanly, and also sweet like the soft/juicy/stringy ones would be awesome. You wouldn't have juice dripping everywhere, get crap stuck in your teeth, and end up with a pit that would make your hands messy when you were done.
I hardly have room in my backyard to devote the rest of my life to growing peach/nectarine cultivars, but since it seems like I'm so interested in growing fruit, maybe I should try somthing smaller like berries. Berries are very temperamental, though, and it's something I'm sure would be ravaged by the local wildlife in no time. Boo!