The problem is that cattle will walk off your nutrients..... like graze a while then go stand in the shade or the pond taking their manure and urine with them. That's why I like moving them quickly and clipping at least a couple times a year to put all that residue back down. I think it's similar effect as unrolling hay but better.
About clipping tall pastures now like what you said with blackberries ....too many weeds will close off the canopy and not allow your grass to capture sunlight... clipping to open the canopy would most likely be productive just don't go too short