There's another term "Vra" which is the recommended rough air speed.
In practice this is usually the same as, or slightly less than Va. Often they co-incide, leading to the confusion, although the definitions aren't identical.
The rational is that at Va you are at the highest speed at which a stall will occur rather than an overstress, so it should give as you say the best protection. If you do stall an L1011 in turbulence, do let us all know, it should be worth a few beers!
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