Its more professional definatly, i have got quite a bit of work when neighbours etc have just noted my no. Or email Down on the side of the van and called me a few months later
but its more than just that really, is about your company awareness, i personally have branded work wear, signwritten van, website, company email, a logo and plenty of business cards
its all part of your image and really helps with your overall marketing, and makes you look like an actual business rather than just a local 'sparky' in a van, i think people feel more comfortable that way as your more tracable, and i feel i can warrant a better price
its good for your business when your van is always seen driving round your local area, parked out side different houses/businesses, believe it people do notice you!
as for break ins to your van, i think that ANY van left in a vunerable place will get broken into, if the wrong kind of people are about, regardless of the writing on the side, get a van vault and extra locks on the doors
to me an unwritten van would look like someone who does jobs on the side/part time alongside employment and sometimes may put people off, and at the end of the day your just another white van driving about