I've encountered situations (including when playing against the AI) where there was no town portal available, and I needed to move the main hero across half the map to defend a town. In such cases, the so-called "unnecessary" diplomacy, combined with three "unnecessary" diplomatic artifacts (attacking another hero and then retreating with your army), proved to be quite useful, as without it, you could be left completely defenseless, or even without the possibility of doing so. So, it's up to you to decide...
As for the necessity of diplomacy in tournaments, I think that if Alan includes a map with a skeleton converter on the third stage of the skeleton collection tournament, the answer to this question will become obvious.