For controversial forks

The possibility of controversial hard forks prevents any group from being able to force their will others
See Meni Rosenfeld's post and essay, Jeff Garzik's post, Justus Ranvier's post, and Mike Hearn's essay.

A rule that a controversial hard fork should never happen gives any group who wants to maintain the status quo veto power over the direction of Bitcoin, and allows such a group to impose their will on the entire community. Their power is constrained in that it consists entirely of keeping things exactly the same, but it is absolute.

When a hard fork occurs, no one is forced to do anything. All users / miners / businesses are free to move to the new chain or stick to the old chain. If most users migrate to a new chain that is less decentralized yet some people still value the more decentralization-friendly old rules, no one stops them from remaining on the old chain. Things do become less convenient for these people because the value of coins on their chain will drop, and fewer people will accept their coins, but that's just a consequence of everyone being free to make their own choice. The people on the old chain never had a right to keep everyone else on that chain against their will.

Clearly it is better for everyone to agree on a hard fork than to have a controversial hard fork, but when people can't agree controversial hard forks are the only way to resolve the dispute that does not involve one group imposing their will on another.

For a more detailed exposition of this view, see the Market Governance FAQ.