Q: How good does a scotch have to be before an MB won't put it in a Rob Roy and insists on drinking it neat?
A: Conrad, you've asked what seems to be a simple question, but this would send a group of professional bartenders into a quarrel which undoubtedly would need to be settled over a few drinks and possibly some wrestling -- the best way to resolve any disagreement.
As we've mentioned in the past, the toolbags of this world think ordering Dom Perignon during the holidays is a sign of class. They're also probably the type who might order a Courvoisier and Coke. The MB knows how to toe the line (even with a law enforcement officer watching) between ignorant excess and the cocktail version of artful dishevelment.
Some "mixologists" (including Gary Regan), simply think that price should be your guide: if the scotch is north of $100 a bottle or so, you should drink it neat. Fortunately for the MB, you have your own taste and two simple rules as your guide:
1) It's fine to use a scotch up to 12-years old in a Rob Roy
2) Avoid using any scotch that's too peaty (think Laphroaig or Lagavulin)
Of course, if you love the peaty scotches you might be able to find the right blend of sweet or dry vermouth (remember the MB likes his Rob Roy perfect or fully dry, with onion garnish) to make a more-than-acceptable Islay Rob Roy. In this case though, you better have your A-game on, or your date may be calling you "Old Swampy" when you try to get more than just cordial.