If spending is allowed to increase on a structural basis, but higher revenues turn out to be transitory, the budgetary position starts to deteriorate as soon as economic conditions turn nasty. This was exactly the mistake made by some countries in the beginning of this decade. Let’s not repeat that mistake.
Countries with sizeable deficits and public debts should use the current windfall in tax revenues to consolidate. Sound public finances are also crucial for dealing with the costs due to population ageing.