Do you have a link to the reliability statistics?
http://www.nytimes.com/gwire/2010/05/04/04greenwire-warnings-on-backup-systems-for-oil-rigs-sounde-30452.html
In this article from the NYT, they allude to it being expensive and there being "insufficient data" about it's reliability in certain circumstances. That's not the same as "systems fail more often than they work". I'm imagining that it's tough to get the data since that would be either very expensive or the testing conditions are hard to simulate. That being the case, should there still not have been a mandated secondary form of shut-off valve? Judging by what I read, that's both a necessary and prudent thing to do given the nature of the circumstances under which these kinds of drilling platforms operate. Oh... but wait. That would take away from Dick Cheney's profits at Haliburton. The company that just happens to operate the drilling rig that blew up...
And if they're so awful, why do two other major offshore oil producing nations, Brazil and Norway, require them and use them?