I think there should be a pref button to turn this feature on and off. Or a startup command that would disable the feature.
I accicentally open up blocks several times a day. Usually its from hitting the right mouse button for an "enter" or "repeat last". It takes a minute to open the block because a new instance of Rhino has to launch - slow. I never open models by double clicking on them. It just takes too long. I keep 3-4 instances of Rhino open all the time and drag-n-drop to open files. Much faster.
So, for me, the "double click to edit block" launches instant desire to smash something. The other 3 guys here don't seem to have the problem, so maybe it's just me...
For the couple times a day that I actually want to edit a block in place, I'm just fine with the BlockEdit command.