I hope one day I have business worth defending. I have read plenty on the costs of defending and pitching new business. I have seen Agencies walk away from contracts worth hundred's of millions. You only make that decision if the Agency-Brand relationship is dysfunctional or antagonistic.
Often by that time it is hard to repair something that is broken. But that is not always the case.
It would be bad business practice for a company spending tens to hundreds of millions a year on advertising not to look at other Agencies or at least hear their pitches/advantage claims. You always want to have best fit and keep your Agency from becoming passive. When this occurs there is a good chance the incumbent will keep the business. If your operating honestly and in the best interests of your client all the time, there should be a good relationship between the two parties.
Whether or not Mindshare not only defended their Unilever account for the US and gained Canada Media Buying duties as a strategy or lucked out, it should be strategy. A review should be looked at as a prime opportunity to not only defend, but increase your market share with that client. The major advantage is while the competitors will be showing what they 'would' do for your current client, you get to show everything 'you have been doing' for the client. And if you have been doing a great job this is the perfect time to ask for more business.
Think about the potential to use guilt ethically and honorably without much effort. Your being asked to spend money to defend business your excelling at. Your client knows this. And if your working together properly they will feel like a partner, who is being forced to look around. Kind of like your dating someone who insists every 2 years to date other people just to ensure they are with the right person. Then when they realize they are they feel bad and buy you gifts or take you out to make up for it.
Obviously Agencies are not pleased when they are requested to defend in a 'review'. They know whether they have been performing and if they have been, this is a great time to gain business.