If you’re uncertain about a term used in this post, please check the Terminology page.
Someone recently asked why throwing to the outside is wrong. They correctly pointed out that throwing to the outside on a lefty draws a somewhat predictable return and that there is potential to capitalize on this predictability; the righty can throw to the outside, block the predicted return, and riposte for victory. Therefore, let me address the question of “Why is your default advice to righties not to throw to the outside?”
There are a few answers to this question. The first, and most basic, is that “Don’t throw to the outside” is good beginner advice, like “Don’t start a sentence with a preposition.” Good fighters, like good writers, learn the rules, and then actively choose to break them for specific, intelligently-chosen reasons. You can throw to the outside, but you’re breaking a basic rule so you better have a good reason for doing it.
The second is that it is simply dangerous. Reactive actions are faster than initiative actions. Worse for you, the lefty is accustomed to this exchange and likely has it drilled to a higher degree of skill than the rest of his game. You’re throwing his favorite combo. Generally you want to avoid giving people their best-case scenario and look for weaker areas in the fighter. So you shouldn’t just do it willy-nilly. Of course, as you get higher-skill lefties, the skill-variation between this shot and other shots narrows as they equalize their skill at all areas of their game. Likewise, as the righty grows in skill, this becomes less risky simply because he knows it is risky and has planned accordingly.
The third answer is that you can use people’s habits and reflexes against them, and in that scenario throwing to the outside is acceptable. This gets into the “chess game” of fighting, where it is not just a matter of physical ability, but of mental preparation and planning. You plan to throw to the outside, draw the inside return, block, and riposte for victory. That works, until the lefty plans to draw a shot to the outside, block, counter to the inside to draw a riposte, block, and then riposte for victory. That works until… you get the idea. Of course, the other guy may not follow into your plan, so mental flexibility remains important.
So, in brief, “don’t throw to the outside” is a “combat rule.” It is permissible to break a combat rule, but you need to know what rule you’re breaking, why you’re breaking it, and what the repercussions of breaking the rule are.
Addendum: Brett pointed out the value of off-timed shots, and made an excellent point. I have included his comment, slightly edited for terminology.
I don’t think you take into account off timed shots. Assuming I am sword and board, I usually throw outside to trigger a predictable riposte and kill them in the riposte. I’m talking about an outside beat into a counter attack. I don’t want to block and then exchange; I want their reactions to be automatically wrong.
 Andrew E. Welchman, James Stanley, Malte R. Schomers, R. Chris Miall1 and Heinrich H. Bülthoff. “The Quick and the Dead: When Reaction Beats Intention“. Proceedings of the Royal Society B: Biological Sciences. 2010 Jun 7;277(1688):1667-74