I will play the middle ground and agree with both Brad and David. Both have their place in a professional detailers arsenal.
I have worked with just manual labor, a pressure washer, an extractor, a steamer, and currently all of the above daily. Use what you have to the best of your ability to get pro results. If you add another tool, it's just another option you have and you will learn which situations call for what.
Here's a breakdown of a few of my personal techniques:
Scrub
For light soiled carpets and upholstery I will mist the carpet with an APC 10:1 solution. Then, scrub with a carpet brush, wipe with a clean MF, and vacuum clean. Dry.
Scrub, Extract
For most carpets, this is my go-to combo. Mist the carpet with an APC 10:1 solution. Scrub with my Cyclo and hit the angles with a carpet brush. Extract with clean water, wipe with a clean MF, and vacuum clean. Dry.
Scrub, Pressure Wash, Extract
This is my adaption from David's use of a pressure washer for heavily soiled interiors (I think the thread was a minivan?). I use this for heavily soiled floor mats or disaster interiors. Pre-spray the carpet/upholstery with the pressure washer (no soap). On floor mats, spray until water comes off clean. Mist with an APC 4:1 solution then scrub with the cyclo. Extract with clean water until it comes off clean. Dry.
Scrub, Steam
This has replaced my method for most vehicles under 3 years old or moderately soiled. Mist with an APC 10:1 solution, scrub with the cyclo and carpet brush, and steam with the 3" brush to scrub hard spots or stains. Finish with a clean MF on a 4" steam brush and vacuum clean. Dry. For very light soiling I will just use the MF and 4" brush with a mist of APC.
All these methods work, in my experience. Some take longer, some are more effective in certain situations, but you can use what you have to get the same end result. However, I would never just use my extractor without scrubbing first.