You may be looking at it a little too scientifically. I think giving AFVs 'hull down' status when on higher elevations is a way to abstract the ability of tank commanders to use the protection that slopes offer. They'd move tanks into a postion where less of the hull was exposed. In addition, units firing from higher elevations may get top hits.
Therefore hills offer a defensive and offensive advantage. In this way the game makes hills important.
Cross