It de-values your work. If a customer thinks you're ready to work a house for free, then it can't be a job worth paying for at anything over a couple of quid.
If someone cleans my windows and takes 15 quid, then goes and does my neighbour's house for free to show his work off, I'd think that if he can work for free, why is that sod charging me 15 quid!!!??
There are millions of potential customers out there who pay upto double for a first clean, why clean someone's windows for free?? Rather him than me.