It could only be to get him more involved - the same reason Gallen plays in the middle (when he does).
But I agree, I think if a forward is good enough to play on the edge then he should. Getting more involved in centrefield is pointless when there's more big merkins in the way.
Look at how much less effective Thaiday is playing in the middle. Sure he makes more runs and more tackles but a big fast merkin like that with linebreaking ability is wasted in the middle of the field. The Broncos' problem though is that they have plenty of quality edge forwards but not enough props. So when they pick their best 17 and fit them into a team, Thaiday has to end up in the middle. They should have let us have Gillett and bought Allgood, and big Sam would be back to his rampaging best. They might even save a couple hundred grand a year.