I use groups for things that would be irrelevant to show in the component browser. I even make duplicated objects groups, if I know I wont be modifying them again. E.g. a table leg can be a group in an architectural project because the table as a whole is the component that I might later pick from the component browser, not the leg.
Contrary to popular belief duplicated groups share the same definition, just as components. The difference is that groups are made unique as soon as you modify them while components requires you to actively make them unique from the context menu. You can try yourself to copy a group a few times, select it and look in Entity Info how many there are in the model. As long as you don’t make individual groups unique and end up with duplicated identical definitions there’s no file size benefit in using components either.
Even if I change my mind and realize I want to edit a duplicated group in all instances there is ThomThom’s Selection Toys to convert a group to a component.