The distribution of sword/armor choices in an MMO will not be the Nash equilibrium with overwhelming probability. In fact, it probably won't be anywhere close if the choice is at all complicated.

Correct. Do you know what the distribution is? Can you gather statistics? Do you understand the mentality of your opponents so well that you can predict their actions? Can you put the game in some reference class and generalize from that? If any of the above, knock yourself out. Otherwise there's no justification to use anything but the equilibrium.

To expand on palfchristiano's quibble with "if you don't know exactly what is going on, you should just play NE": The technically correct phraseology for that part may be too complicated for this post. But it would be closer to "if your belief that you can predict your opponent's non-ideal behavior, multiplied by your expected gain from exploitation of that prediction, exceeds your expected loss from failing to correctly predict your opponent's behavior, go ahead."