The logistic requirements for swapping deployed sqns is minimal - a single movement of 100-200 pers being the sum total for a formed unit move such as practised by Tornado sqns, or a steady drip-feed for non-formed deployments such as the FI. The aircraft, tools and spares all remain in situ. That small logistic requirement is a small price to pay for the goodwill of our people.
I think that 4 months is already too long for some kinds of deployment. Deployed operations will only ever involve part of the skill-set for which a sqn trains - FJ sqns in Afghanistan, for example, will not get any routine low-flying, radar SAM evasion or air-to-air work for the whole time they are deployed, and the rate of weapons employment is low with a narrow range of options available. In-theatre training is impossible, with the sole exception of a very limited strafe facility at the KAF range. After months of this, skill fade is a serious issue - would you like to be a JTAC danger-close to a target being strafed by a pilot who hasn't done a pass in 6 months? You can mitigate this to an extent by rotations for simulator training in the UK - but then you might as well rotate sqns instead. Afterwards, sqns need a significant period of consolidation to recover other skills. 6 months would make this problem even worse than it already is.
The standard TELIC det for the Tornado force was 8 weeks per squadron. After such a brief hiatus in normal sqn activity, training for contingency roles could easily pick up where it had left off. Not the case any more, where the longer dets (with their associated extended post-op leave) come to define the whole year - particularly when the amount of pre-deployment bo**ocks now adds up to about 2 weeks of no-fly!
Many of us would not have joined the RAF had routine 6-month deployments been part of the deal; those who chose to join the RN did so in full knowledge that they would be away for a significant proportion of their time. There are upsides to the RN lifestyle - a degree of geographic stability for families for one, which is emphatically NOT the case in the RAF. Our downside is a 2 - 3 year posting cycle to locations in all corners of the UK, and postings between front-line / training / staff tours virtually always require a move. It looks like the Army will be joining the RAF in this lifestyle as they take over our old airfields!