> The concept of everyone using a common, well-audited contract makes sense though.

Just Parity's contract was neither common nor well audited.

> Virtually all Bitcoin multisignature transactions use the same script construction and it has never been hacked, despite being much older.

The only multi-sig issue that I'm aware of in Bitcoin was the Bitfinex hack but I don't know in what relation BitGo's multi-sig implementation is to the standard multi-sig.

The EF multi-sig implementation has been audited and AFAIK there has also never been an issue with it.