Purely Leptonic B decays at High Luminosity e+e- B factories

High Luminosity upgrades of the KEK-B collider are being discussed. We consider the role of the purely leptonic decays B^\pm -> l^\pm nu and B^0 -> l+l- in motivating such an upgrade. These decays are very sensitive to R parity violating extensions of the MSSM, and we show that future runs of the KEK-B factory can be competitive with high energy colliders for probing such models.