To answer your question. Very dirty ones get more agitation, pre-spray dwell time by the fact that they get more agitation and a good 12 flow flush. TACT?!!! So far (26 yrs) worked all righty and clients seem to be quite satisfied with the end results. Since I don't specialize in trashed carpets, I don't come across the trashed kind too often, which brings me to the next point.
Which is, that I am sure the brush head will do a fine job and save time on the trashed carpets. However, it is absolutely not necessary for a simple every job clean, as pushed here so frequently lately. IMHO: It is a specialized tool that should be used for specific circumstances. And if one has enough such jobs, well worth it.
One concern would be: When combining high heat and a brush on a rotary, it may (in my opinion again) carry the risk of untwisting some staple fibers, particularly if done often and for a prolonged time in certain areas. And even more so with low heat set carpets. Once blooming happens besides the change in look, it will also mat substantially more in these areas.