> But I think the decision scales should be roughly the same if the sampled training sets used for each estimator are roughly the same size.

Even when they may each have been built on different feature sets? I am really not so sure about this strategy. In any case, I wouldn't implement it as the default behaviour of a bagging estimator. This doesn't look like something standard in my opinion (in the context of ensembles). 

> BTW, would it make sense to add an option to force using voting even if the underlying estimator supports predict_proba?

Results might be a bit different, but not that much. We have discussed this earlier in this thread and my fear is that it will complicate the interface for no major improvements.