Easy Street.
When you say "If it's firepower the army are after, why don't they make better use of the stacks and stacks of air assets (including AH) armed with precision-guided weapons that orbit daily over Helmand? Given the small distances involved, there's always something within a few minutes' flying time, which means that the required 'smack' can be laid down in much less time than it takes to roll out a Chally - which at the end of the day is unguided artillery on tracks..."
Do you actually have much knowledge of the Chally2?
You make a fair point about the use of air power to react to situations, as long as you are happy for folks on the ground to have to wait many minutes longer for their fire power support. Have limited support, which will run out of fuel sooner or later (as will the Chally, but its 24 hours on station is somewhat useful, and almost limitless in a 'mobile firebase). The main gun on a modern MTB is also very effective for many uses in our current theatres, plus the baddies will often not be able to see the MTB to be able to fire back at it with RPG's etc (and lets face it, the Chally is more able to cope with RPG's than an Aapache!).
The British army has a good experience of using MTB's in fairly static locations to provide fire support, and as far as I see this, it's a win win situation for everyone.
Folks on the ground get added firepower on call (which is very accurate, even against moving targets). We get to save money by using cheaper ordnance. We also end up with air assets more able to concentrate on more distant attack and MERT cover.
The downside is that thanks to Noo labours penny pinching, the Chally2 is not as good a sandy environment weapon as it could be!
All just IMHO of course.