Have you seen this advertised:
"Clean JUST LIKE A PROFESSIONAL with this rental machine!"
There are many people that believe this marketing about store-bought or rental carpet cleaning machines. Sadly, depending on the company you hire, it may be true! When its just simple maintenance cleaning, ie light soil, yes, these machines can do an adequate job.
When it isn't simple, ie heavy soiling or other, then these machines are drastically lacking in ability. They lack the performance; there isn't enough UMF in either the machine or the cleaning solution. The cleaning solution is not able to emulsify the soil so the operator goes over and over and over the carpet, the machine lacks the vacuum to recover the water resulting in over wetting, saturating, leaving excess soapy residue and often a flooded carpet which takes days to dry. Over the years the hardest carpets I've had to clean are ones cleaned by home or rental units.
A real professional however, has invested much time and money into the proper equipment, cleaning solutions and training on how to use them. A professional can clean lightly soiled to extreme soiled carpet and upholstery without causing further damage. You can't "rent" that kind of expertise.
Having a home cleaning machine can be a good idea however. You can catch those accidents quickly and do a little maintenance cleaning. This is the best use of these machines. Then once a year have it professionally cleaned.
The Best thing about having someone clean the carpet for you... IS that someone else is cleaning the carpet for you!!