On the positive side, it should stop any potential for argument if something major goes bang during the first two years. Whether that justifies the additionalcost is a risk-based decision only each person can take individually. As has been said, if it does go wrong and doesn't have dealer service stamps, you can probably expect a battle.
On the negative side, I feel much more confident in the quality of work of my local specialist than in some dealers I have used (which includes Battersea/Park Lane). I'm just out of 2-tr warranty period now, so I will continue to use local independent.