When turning off auto-combat, there are movement and fighting advantages to each unit, hoplites, horse troops, and rabble. Having a mix of units gives you a larger pallette of moves and tatics to use.
I doubt though that many people will achieve an all horse army for all twenty units. That'd take 160 horses + 80 reserves = 240 horses total. And having at least 40 elite houses all on the same map, all eating double rations of wheat. I'm doubtful that many Zeus maps have the realestate to support this large of a city, plus the wheat fields necessary to feed all those horses.
Even with six horse farms, all burning wheat at an enormous rate, it would take ten years to achieve an all horse army. And never use your precious army for that ten years, or else you'll take casualities, which will set you back even further in your timeline. Take 2 casualities per unit, an average result of winning a battle, and now you're set back an additional 40 horses, or almost two more years, all just from one battle.
I'm sure academicly the 20 standards of horses army can be done, but only by dragging one's feet on when to fight and drawing out an adventure far past when all enemies are conquered. I personally find that kind of game a bit dull.
[This message has been edited by Innovandora (edited 11-30-2000).]