The guys going in cheap to get the work are often doing us a favour in the long run. Some will stick with it and in time will offer a professional service at a sensible price,.. the others will move on to the next adventure. Either way, they'll be getting new customers who would never have considered using a window cleaner before, and will probably find the service invaluable in the long run. These new startups are expanding the market for us,... making sure window cleaning remains a thriving industry for many years to come.
As for the contact details,.. my opinion is that we work in an industry frequented by cowboys/gypsies and dole scroungers. If putting your full details on all your advertising helps you to stand head and shoulders above these, then why wouldn't you want to do it?