The building requirements for stradiots are in place to simulate their late appearance in history, rather than some high tech development. They are the lightest cavalry unit the Venetians get, having same or less armour than mounted crossbows or sergeants.
Their speed makes up for their lack of armour. Once I was able to build them, they always had a place in my armies and it wasn't just to catch fleeing generals (which they do just fine), but more importantly to annihilate enemy missile units. As Venice, I relied heavily on my Pavise Crossbows ability to decimate slow armoured targets, and I used those guys plenty. However, I met enemies like Hungary who built their stacks with at least one third pav xbows, so a shooting match would result in lots of casualties on my side. Charging the front with heavy cavalry only works 50% of the time, as their slow approach gives the enemy time to pull back its missile units and respond with spearmen, or at least shoot another close range volley at my knights. Stradiots are much better for this job, as they have more chances to catch the missile units and get less exposure to enemy fire.