judging by the numbers joeball123 listed, the XP split is nowhere near as devastating as people believe (assuming the chart is at least close to accurate)
let's just do a few examples, shall we?
say you have encounters worth 1000 XP in your immediate area that you can defeat (let's ignore the implications of whether it's harder or easier to do it with multiple heroes)
doing that stuff with 1 hero gets him to level 10 (almost 11)
doing the same stuff with 2 guys, splitting the XP in half for each of them gets both to level 8 (halfway to level 9)
splitting between 3 guys results in 3 level 7 heroes.
big deal, really.
let's do the same with 3000 XP total
single guy - level 16
2 guys - both level 12 (almost 13)
3 guys - all level 10 (close to level 11)
yeah, totally devastating