I find that a pile/grooming brush is fine for grooming the carpet after HWE, it finishes the job really well, helps provide the wow factor for the customer!
But using a hand brush of any sort will do next to nothing to lift the pile and very little to agitate any MS or presprays!
I use a sebo to agitate One Step as well as lift the carpet pile, it dramatically improves the job we do. In realising the improvement that the sebo generates as a agitator and pile lifter, it encourages me to go for an envirodri or similar, it can only take the end result to another level.
cheers
Paul