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it just fits to the common concept of listeners. In vaadin you always see a list of listeners instead of a single one. The upside is simple: you easily can extend your application if you need another listener. Also it feels more native if you see a addXYListener method when working with vaadin. The dowside of course are the lost resources. But I think you can ignore that mostly.
Some MVP techniques use multiple presenters (such as mentioned here), in which case storing a collection of listeners (observers) would be necessary. I'd think it's a good practice to not limit such needs if it's not too much trouble.