At this stage of TNG, Riker might not have completely given up on his quest to do the clever and unexpected. Perhaps ramming at warp does not give you the destructive energies of warp fields (there might be none) or of great momentum (at warp speed, your mass might be zero or less, and your momentum thus negligible) - but instead merely is so exotic that the Borg might not have an optimized defense up to counter such an unusual attack type?
A ship in the process of powering up a warp field to values associated with warp nine speeds might be a "slippery" target to Borg tractor beams. Or a warp field at such a stage of activation could chew up the space around it, not with incredible destructive force but still to some degree, helping Riker penetrate deeper before his antimatter blew. Or the warp flash itself might do some damage to targets in the very
immediate vicinity (as in, touching your ship) somehow.