Why should we think there was no ramming at Wolf 359?
Heck, we basically see a ramming attempt in "Emissary", where that Excelsior class ship dives towards the Cube without firing a single shot. That may have happened a dozen times in the fight, and a dozen other dirty tricks may have been tried out. Nothing worked, because the Borg were invincible; Picard had made sure of that, as he would know all the dirty tricks and prepare the Borg for them.
It's easy to see how ramming would be utterly futile against the Borg. After all, their deadliest weapon in almost every fight has been a super-powerful tractor beam. If a starship cannot break free of such a beam once caught, it is only logical that the beam would stop all ramming attempts as well.
Riker's warp ramming may have been a technique without any other merit besides sheer exotique. Ramming at warp might not be any more devastating than ramming at impulse (and quite possibly would be far less devastating) - but because it made no tactical sense, it would have at least a slim chance of success against an enemy that was prepared against all tactically sensible maneuvers.
Timo Saloniemi