Besides, what more valuable research could be done with them than answering "can we grow crops in lunar dirt" and "what do we need to mix in the lunar dirt to get crops to grow"?
We might want to understand:
The rate at which solar radiation impacts one side of lunar rock vs another (all the specimens were sampled with their orientation photographed). The composition of different lunar materials in different places. The alignment of various internal crystal or magnetic elements to better understand the moon and Earth's history. If we get very lucky, we might pick up a rock that came from Mars, or somewhere else, and we can understand the creation of our own solar system. Solar radiation leaves its mark on the rocks, and provides us a record of our sun's history if we can learn to read it.
We might want to understand whether a lunar regolith sample was capable of sustaining life, but the way we do that is not just grab the existing samples and put a seed in them. They are far too rare and precious. Better to try and understand everything about them and their formation, then try to create as accurate a simulation as possible.
Why is that better? Learning if they can support life seems far, far more valuable and practical than better understanding of its geological history.
If crops can grow in it, then a moon base becomes far more practical, and with a moon base you'll have all the rocks you want for further study.
I.e. priority should go to "what do we need to know to build a sustainable moon base". Understanding solar formation history is of doubtful immediate value.
If we can manufacture lunar regolith, and it's 99.9 percent the same, why use the real stuff (which is literally priceless) ?
> which is literally priceless
The price we'll pay for not using it will be the cost of extra missions to the moon to do those experiments there. We cannot plan a moon base without knowing whether crops will grow there or not.
I'm less familiar with Lunar "dirt" (aka regolith) than Martian regolith, but it's very likely that Lunar regolith isn't capable of supporting life. Martian regolith is a highly toxic blend of perchlorates, oxides, and extremely dry jagged minerals. The lack of a water cycle means that weathering behaves very differently, and the resulting material is considerably sharper and finer than the stuff on Earth. It also, of course, lacks any of the organic content that you'd need to make healthy soil.