Bah humbug! Don't listen to these heathens!
I think I'm probably the biggest (read only) proponent of the MIGHT PREFECT defense...but it fits how I play.
I don't like legionaires...too slow for my tastes. I build all jav's, and cover invasion points with lots of prefectures on a closed road.
The advantages of prefects? As opposed to lions and gladiators, prefects don't need a road to walk to the enemy, they'll charge right off after them. Do they win? Heck no, prefects can't beat a zebra, let alone a carthaginian. But they do delay the enemy, and thats the key. 1 prefect can delay an enemy soldier, holding him in place while dozens of javelins come flying through the air to impale them. Perfect, as far as I'm concerned. I can pull units from the other side of the map before the enemy gets to my city. Towers, as well, get to pick off the enemy while the brave prefects hold them in place.
Other advantages? Prefects respawn much quicker, vital in a long fight. Prefectures are cheap, meaning you can add/remove them to manage little inconsistancies in your employment. I've had occasion to remove 20 prefectures when I needed more food production, freeing up 100 workers...it works pretty well.
Overall, prefects work well with my missile-weapon-favouring tactics. In a bottleneck, the lil guys can clog up a huge army and let me pick them off. When that hidden invasion point pops up, they give me time to run over from the other corner of the map.And given the somewhat small limit of 6 forts, I'm happier having a more mobile force, as opposed to a stronger legion of soldiers. Plus, prefects give me time to line up all 6 of my jav's in the same spot...96 toothpicks flying through the air has a way of stopping even the largest army ;-)