To all you 29er's out there!

Because the wheel will be moving forwards or backwards depending which size cog he's using.

I thought it meant moving the axle forward/backward in the dropouts. In which case you'd need to move the calipers too.

^Yep, that's what I mean. Sorry, possibly not very eloquent of me! Yes, my initial reaction is that a cog difference of 2 teeth may not cause enough forward/backward movement to mean the brake pads need to be adjusted each time, however £84 is quite a lot to fork out for a cog when i'm not certain.