I have a Surly Tuggnut on one side on my Guv'nor. You only really need one on the drive side.
They're not the cheapest but they work well and look good on the right sort of bike. Also you just never know when you'll need a bottle opener
If you think about it, the one on the non drive side does cock all. But some of us like symmetry even on things as asymmetrical as bikes.