Which hero do you like?

It's not always worth filling a new cell with a skill. The more cells, the lower the chance of upgrading the desired skill. Sometimes, it can determine the entire upgrade process in the early stages (better to have one strong skill than two that are almost there).
Although, I wouldn't miss the opportunity to give a nature mage necromancy.

Why not... if everything is already planned, it's better to ignore everything else. Let's say I've fully upgraded Nature. I gain a new level... but the intended Haven skills aren't offered. But if I already had one cell filled with a Haven skill, it would be easier! I agree that this slightly weakens the hero at the beginning... it spreads their experience. But you really can't rely on randomness. What if