Its all about the secondary abilities..
When your summoning an Elemental, its not so much about the HP, AC, attack bonus, and damage as it is about the secondary abilities.
First up, all elementals are immune to Critical Hits, Poison, Sleep, Paralysis, and Stunning attacks. The most important here is the critical hit immunity. Another thing is that they tend to be True Neutral. This means that a Protection from (Alignment) spell is not as effective against them.
Air Elementals:
Mobility: Perfect flying, high movement rate. They also have Flyby Attack.
Whirl Wind: They can impede the movement of opponents, and obscure vision.
Earth Elementals:
Mobility: Being able to go under ground means they can easily get into flanking positions.
High Str: If you need something broken, smashed, or moved, then they can likely do this for you.
Fire Elementals:
Start Lots of Fires: If you should ever feel the need to raze large parts of a city, or to destroy lots of property, this is the way to do it.
Water Elementals:
Sink Boats: This is much more effective then simply attacking hostile pirates.
Vortex: As air elemental, but under water.
While an Elemental may not have as many obvious abilities and bonuses as other summoned creatures, one should not forget the indirect benefits either.
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