Actually, there's a simple mathematical reason.
Let's say right now, Australia maintains 5 sqn (2 F111s + 3 F18s).
If 2 sqns of F18s are procured today, sure, the RAAF maintains a stronger fleet and in fact expands defence spending and pilot requirement because of the new Tiger/wedgetail pilots.
A decade later, when the JSF comes onboard, the F18Cs are not yet due to retire (esp after a recent upgrade program), the new F18s are just within their first decade of service. To introduce the JSF then would mean having to create new squadrons or retire the F18Cs earlier. Its all a question of cost. Doable? Definitely BUT it will be a question of cost.
Now if the RAAF leases F18s in the interim whilst shifting some F111 pilots to helos/AEW, a decade later, it can immediately replace the 2 short sqns with JSFs and look to retire the F18Cs at the end of their natural service lives with more JSF purchases (not having spent $$$ on 2 new sqns of F18s). Ultimately F18s will be phased out sooner without sacrificing pilots.
Just to clarify, I'm not saying that purchasing 2 sqns of F18s is a bad idea or that its not doable. In fact, purchasing 2 new sqns will be a significant boost in capabilities. Just that if cost is a signficant factor, I'm sure the MoD is running the same numbers.
Okay, a few points to go over here. For starters, new F-18 C/Ds are not available, production of the C/D models has ended something like ten years ago. Also, the current Hornets operated by the RAAF are the A/B models. The only F-18s currently in production are the E/F/G Super Hornets.
Right now the RAAF operates approx. 24 F-111C/G/RF-111C in the strike, maritime strike, photo-recon & training (for the F-111 platform). According to current plan, the F-111 will be retired in three years, sometime in 2010, having had over 35 years of service. That will leave approx. 57 F-18 A/B Hornets in RAAF service, these cover the air defence, CAS, strike, maritime strike and training/OCU roles. Also, given the age and wear on the F-18 A/Bs (acquired in the early 80's) after the HUG/center barrel replacement, many will still need to be replaced in the 2014-2016 timeframe.
The idea behind the possible lease/purchase of the Super Hornet is to cover the time when the 24 F-111s are not in service, and as a possible cushion for when some of the HUG F-18s have to be retired if there are any F-35 delays.
It has been mentioned that F-111 pilots could be transitioned to either the Tiger ARH or Wedgetail AEW until F-35s are available, however there are several issues with this.
For starters, the Tiger ARH is an Army Aviation, not RAAF aircraft. It has a role completely different from either the F-111 or F-18. A Tiger ARH is for tactical recon & battlefield CAS, an RF-111 is strategic recon, and the F-18 can do CAS, but that isn't a main tasking. Also, there completely different requirements for operating a fast jet like the F-111 or F-18 vs. helicopter training & operations. Lastly, the Tiger ARH is replacing the gunship configured Army UH-1 Iroquois "Huey" so that itself isn't a new capability but an upgrade/replacement of an existing ability.
The Wedgetail AEW&C is a new capability the RAAF will have, but in terms of requirements for piloting, it's basically a Boeing 737-700 (or is it -800?) commercial jet with a canoe-fairing. It is used to detect & track targets, not engage them like an F-111 or F-18 would.
Lastly, part of the goal is to maintain approximately 80 available combat aircraft for the different roles the RAAF has. Unless either the F-35 becomes available sooner than expected or an interim/additional aircraft is purchased, the RAAF will not be able to maintain an 80 aircraft force until 2014 when the F-35 is initially expected.
To reiterate, the purchase of 2 sqd. of Super Hornets would not really be a boost for the RAAF, it would instead maintain capability between the retirement of the F-111 and introduction of the F-35. It would also ease or reduce potential problems if there are further delays on the F-35 in-service date, since by then many of the F-18 Hornets will have reached the end-of-service life, even with the HUG program.
-Cheers