Marrow is a wellspring of stem and progenitor cells that support a number of systems, like blood, bone, and the immune. It won't be a panacea, but if the procedure can significantly improve quality or extend life, then it won't take much of an effect to make it worth the costs.
Highdea: What if the immune system (stem cell pool) ages similarly to other parts other systems/organs and becomes more "sloppy" with time (supplies the body with sloppy mature immune cells), allowing the aging cells to survive longer than they would if the immune system is "fresh" or young. And by introducing "young" stem cells into an aging system, the immune system would overreact to the older systems/cells/organs and cause their death/apoptosis? For whatever reason, this is making sense right now