I've encountered situations (including when playing against the AI) where there was no town portal, but it was necessary to move the main hero across half the map to defend a town. In such cases, the seemingly "unnecessary" diplomacy, combined with three "unnecessary" diplomatic artifacts (attacking another hero and then retreating together with the army), proved to be very useful, because without it, you could be left completely defenseless, or even not have that option at all. So, it's up to you to decide...
As for the necessity of diplomacy in tournaments, I think that if Alan puts a map on the third stage of the tournament for collecting skeletons, where it is possible to use the skeleton converter, the answer to this question will be obvious.