To be honest who cares about offering exp boosts in their cash shop? Levelling is mostly easy and fast, but above all tedious. This exp boost will only ever help 85-90 because from 1-60 (with heirlooms) you spend most of your time travelling to new areas when you out level the current one. Same for BC, Wrath and Cata content. There's really nothing wrong with this. I will complain the moment they give a 'gold boost' or anything else which gives a clear advantage in PvE/PvP (at a competitive level).