As for what it's used for, which imo is far more important than overdoing scarcity*, NIM can be used for any kind of purchase. Right now you can buy select Nimiq merchandise at the Nimiq shop, and there are a handful of other shops using the Nimiq WooCommerce plugin to sell stuff (stoopwaffels, herbal stuff). More utility for NIM will come over time. * - especially considering the limited supply already makes it scarce
The answer is yes. "would (B) not be able to claim it?" Yes. B would not be able to claim it. You can only claim if you held coins at the time of the snapshot. B didn't get the coins until after the snapshot and so gets nothing.