YES
free advertising targeting an area you already work in whilst you are working !
no longer are you some dodgey bloke with a hose pipe hanging out the back of his van,
but a window cleaning COMPANY.
If the above is true or not I shall leave for others on here to interpret but in my opinion it makes you more approachable to potential customers ( they are more likely to come over and ask for a card or quote) and over time your van becomes a familiar site around town.
e.g. "I'd been meaning to ask you for a quote, I see your van around a lot"