Relational fields with attribute 'invisible' set to true are in views because they are used in domains or contexts. In this case their relational data is useless, only their ids matter. Before this rev., their relational data was fetched anyway. More precisely, it was the case for many2ones after a default_get, and for x2manys with inline views, or without view (e.g. with widget 'many2many_tags').