We might not be very impressed by his see-through motives - the UK defence budget is under extreme pressure at the moment - but Liam Fox is fundamentally right when he says in a leaked letter that it would be folly to set the 0.7% of GDP commitment to development spending in law. This is the kind of rigidity that we march around the developing world advising against, so why shackle the budget priority setting process at home? DFID needs to make it's case in the expenditure round just like the other ministries.
Of course, his case might be rather stronger if the Ministry of Defence hadn't made such a balls-up of the strategic defence review, which seemed to be as far away from being strategic as one could get!