While people would have the option to avoid such a thing (for example, for religious or ethical purposes), it would have a much higher chance of saving another's life.
The better system would be to eliminate the ability for people to not answer. Right now, no answer means no, whereas in the ideal system, answering would be mandatory, eliminating the need for opting in or out.
Opt out organ donation puts people at a higher risk of dying in hospital*. It's not generally considered just moral or fair to put people in greater risk without their consent.
Opting out is a presumption of consent. For something as vital as vital organs (heh) there must be freely given consent, not simply presumed consent. If presumed consent is okay for harvesting organs, why is presumed consent so uncommon in other aspects of life?