Part of that is true, more of the brine reaches the resin.. but if the resin is aloud to move even a little out of the way of the flow then it may not get the charge when it should.
Think of it this way.
When the water heats up the water that water is going to expand both to the facet and back down the feed line or the incoming line to the water heater, if there is a softener or other water treatment in line before the water heater there is going to be an Unpacking of the media bed and let untreated water slip through and into the rest of the house and the water heater, now if that can happen,... it can also happen when trying recharge the resin. The resin needs to be packed for the up brining to work best, if that bed is aloud to unpack even a little the effect of brining is lowered.