1. The probability of your being in a stressful situation without the option to leave is high - you probably arrived via plane, so you can't simply go back, and you don't know the local laws well.
2. You usually know that a customs checkpoint is upcoming.
So, in that case, it's far better to prepare (i.e. don't bring things you don't want searched/compromised) and cooperate.
I was once "detained" whilst going from France to England while the customs official searched my bag.
I complained to the UK immigration and the same customs officier called me back, searched my bag again, and said "unless you agree to withdraw your complaint, we are going to have to continue searching your bag until the train departs and you miss it".
i.e. costing me about £150 in expenses.
As expected, I withdraw it and went on my way.
However I now make a point to record the name / number of custom officials I make a complaint to -- in case they turn out to be jerks like the UK one was.