Rock and soil are not as permanent as they seem. Over time - decades - centuries, they change shape, erode and move about. Earth elementals can do all that in seconds.
As with most level 2 and 3 summoning spells, the number of Earth Elementals you can summon with this spell probably isn't enough to really affect the battle. Don't cast this unless you're short on melee support and you don't have any better summoning spells available.
Earth elementals can be effective in a defensive role - they screen well and will absorb plenty of abuse, both physical and especially magical. They can also inflict fairly good damage if your opponent brings the fight to them. Cast Snake Strike on them to improve the strategy.