The use of cavalry in an offensive role seems to have slowly developed from about 200 onwards, until, by the time of the Battle of Strasbourg (360), Infantry were starting to act as support for cavalry, rather than the other way around. The medieval age of cavalry was just around the corner, and at Adrianople in 378 the Old Roman Infantry was defeated for the final time.
I understand, Macro, that your discussion mainly focuses around the army of the Principate, and so your comments about a lack of native Roman cavalry is valid. It was also still very much the era of the mediterranean heavy - armoured foot soldier.
For us late - period enthusiasts though, the army of the later Empire is just as Roman as anything that went earlier, and they treated their cavalry very seriously indeed. Gaulish and Pannonian cavalrymen, whose families had been Roman Citizens since the early third century, considered themselves as Roman as anyone else from within the Empire.