Well, if you're on top50, you can duel yourself. You're right in the sense that it helps in seeing how AI plays your deck. For example, many top50 people used mono
shrieker rush to farm top50, since even if they meet themselves, the computer sucks in playing shrieker rush (burrowing).
You *could* open two browsers and log in with the same account, then pvp duel yourself, but I don't know how it would work out. You could make an alternate account, farm it enough, then make your deck and let the two accounts fight. However, I'm not sure but I've heard from somewhere that the system sometimes catches that has hacking/exploit, and you could potentially get auto-banned.
So, the conclusion is: don't try it.