Definitions are arbitrary, but if two people define their terms differently, they'll talk past each other a lot. It's not true that two educated people can always communicate smoothly if they both just use the same dictionary, however. Discussing anything complicated or subtle always requires both speakers to select the same basic elements from the dictionary, and then work together to assemble the standard elements into specialized tools for the problem at hand. Meaningful discussion requires a lot more of this kind of preparatory work than one might expect. If two people have large vocabularies it can help them to hit the ground running in a discussion, because they'll be able to use a lot of specific terms and know just what each other mean. It's naive to think one can avoid all the hard work of agreeing on terms by just relying on a dictionary, though. That's like expecting a box of spare parts to jump into your car.
Over-emphasizing the value of dictionaries can even be sinister. Most words have multiple possible shades of meaning, and if you shirk the responsibility of clarifying which shades you mean, but instead throw this responsibility onto your readers, you do more than just save yourself work. You give yourself the wiggle room to switch from one shade of meaning to another without admitting it. You can even build a sort of logical Escher staircase of invalid arguments whose flaws are very hard to pin down, because at any one point there exists an interpretation of your words that makes sense at that point. For your argument to make sense at another point, though, your words will have to be interpreted differently; there is no single, consistent way to interpret your words that will let your whole discussion make sense. But if you can get your readers to believe that any possible confusions will be relieved by consulting a dictionary, then whenever they hit a point in your argument that doesn't make sense, you can just keep on sending them back to the dictionary, until they find a way to squint and twist until that point makes sense. Then on to the next point; rinse, lather, and repeat.
It's hard for most people to notice this kind of semantic runaround, and call shenanigans on it, unless they've had a fair amount of academic experience, just like it's hard to chop vegetables fast unless you've worked as a chef. It's not all that hard to be the one giving the runaround, however. If you're a glib writer talking to impressionable young people, I bet you can count on most of them just getting worn out, after enough trips to the dictionary, and deciding that you must just be smarter than they are.
