I wouldn't use the same brush for agitation to also groom the pile after cleaning. They can get manky pretty quick. Tried it once when the brush was cleaned and looked like new, but after a couple of strokes I noticed wee hairs and lint etc on the freshly cleaned carpet.
But yes a brush is more than adequate to agitate pre-spray into carpets. I use one daily, in fact I did a dirty Indian carpet on Saturday night. Large area, several areas of blacktop. All I used was a carpet brush. As well as hot Enzall, formula90 and a Airflex Storm!
. Client was very happy, do it once a year and have been cleaning it for 4 years.
CRB machines will save you a bit of time but the main advantage is till be a lot easier on your body (less fatigue).