I think an intervention is often a great tool - primarily because it teaches the family what their role in the addiction is. I haven't been through an intervention, but when I sought recovery on my own I became aware of the ways in which I was perpetuating my sister's addiction even though all of the things I was doing were in a desperate effort to stop her from using. I had it all backwards and I'm still shocked at how much power I thought I had.
I think the family making the decision to attend Al/Nar-anon would be equally as effective, because I think the greatest benefit of the intervention process is the family education aspect.
In my case I started recovery but the rest of my family did not. So while I no longer enable the addicts in my life, I can very clearly see where my family is still enabling them, and still actively participating in a huge web of chaos and drama, and even though it makes me very sad, I've learned that that's their decision to make. In our case it seems like an intervention could be good, because it could help educate the whole family on their role in this - but at the end of the day they would still have to make the decision to embrace and work their own recovery just as an addict must do.