I'd argue that Swann is the only consistent, World Class spinner that we've had in my cricket-watching lifetime which began in the mid-80s!
At some point, I think you just have to accept that it's pretty damn tough to be a spin bowler coming through in England and that until another anomaly like Swann appears, England needs to win with seam (as we just have done, spectacularly). And just have our spinner serve as a mostly change of pace or defensive option - and hopefully be able to bat a bit as well.
In that respect, Leach is noone's idea of a world-beater, but he's got as good a record as any other non-Swann England spin bowler in the past forty years. He's not been great this series, but would be harsh to drop him unless you really believed there was someone special coming through.