If you ask me, humanity could really use some unifying influence right about now, for example to help tackle the issue of climate change...
One world government is bad because it will attract evil elements who use it to place all peoples under their control/taxation/exploitation, with no recourse or escape. All in the name of helping people and fighting for <insert your favourite political cause>.
By analogy, it might seem more efficient if families were to live in communal dormitories instead of their separate houses. In reality it would create stultification and at worst mass suffering when individuals took control over all aspects of other families' lives.
However you can ruin the efforts of others by demanding that their work serve a primary political agenda.