Must admit I personally have only ever considered considered it an extra service I can offer. Mainly do carpet & upholstery cleaning and some post tenancy cleans.
Never expect any work from it untill March/April time and last 2 years never done more than 20 jobs over the whole summer, but this year reaching that mark already and its only June.
Would definately recommend offering another service for the quieter winter months, preferably inside work.