Only the dead have seen the end of war - Plato
Succesful defence planning requires two things - a crystal ball and a magic wand. Not sure who I am paraphrasing, but I think its a good quote.
Jimlad - The obvious answer to your question is - the Falklands. After 2006 the Argentines may say an opportunity to do something that Argentine governments have been promising to do since 1833. Whilst the forces (including Tornado F3s) provide capable defence, they would not stop a determined attack. Such a crisis would demand maritime reinforcement of the defences - and involve similar threats as those encountered in 1982. According to the press, the Argentine armed forces (including the naval air arm AND air force) are re-equipping. Argentina is still potentially unstable.
And Sierra Leone was a UK only operation. There was no need to deploy aircraft in roles other than recce and frightening the rebels - but what if there had been? What if the rebels had been backed by another state which took exception to the presence of UK forces and started using their MiGs/Sukhois/whatever against our helicopters - the Sea Harrier would have been needed then. What if the same nation took exception to the presence of British warships and Marines? What if they had blockaded Freetown?
The above is obviously hypothetical - but similar things might happen in the following decade. Both the above are examples where the existance/deployment of the SHAR would have an important deterrent effect. The same could be said of protecting UK maritime trade from third world dictators trying to make a name trying to make a name for themselves by harrasing/attacking merchant shipping (as Gadaffi did). It might be said that the Britain is more likely to be targeted in this we stand out as a major Western power but without the USA's capabilities.
Is it not possible that the Sea Harrier acts as a deterrent to potential aggresors and that this is as important as its actual deployments?
As BlueWolf points out, the only thing we can predict about the future with certainty is that we can't predict it. With a unstable dangerous world and a Government that is not adverse to using the Armed Forces (doesn't like funding then though) the chances of crises involving the UK seem high.