Those quick half-lives decay right away, losing a neutron, right? So that Berillium-11 just decays back into Berillium-10.
The problem is that the blanket is constantly absorbing neutrons from the fusion reactions, that's it's job. So despite using simple berillium 5 to build your blanket, you end up with these heavy isotopes over time, and because the heavier ones quickly decay into lighter ones, you basically end up with a whole lot of berillium-10.