Good point.  It is not needed for AdaBoost.  The reason it's here is because I sometimes use the scikit-learn Pipeline as a sort of glue to combine different kinds of models from other packages.  For example, I have some wrappers that I use with the statsmodels GLM models to make them work in the Pipeline.  The GLM models can take offset or exposure parameters when making predictions.  So, it's not really needed for scikit-learn by itself but has been a useful extension for me when combining scikit-learn with other packages.  It may be out of scope.