Ok, here is the explanation:
Up to now I stated that the quotes are always needed. This is wrong (I said this deliberately!).

The standard does say the quotes are needed as soon as the URL contains any characters which are not letters, numbers, dot ., colon :, hash #, numbers or slash / (I think I got them all, but do not beat me if I forgot one or two characters).

Joe thinks it is better not to use quotes as bad things happen if you forget one of them. I think it is better always to use them because in certain circumstances they are needed, and if you are used not to quote you tend to forget them when they are needed. I normally use an editor which allows me to insert templates (for links, text coloring and many other things). I wrote this template to contain the quotes, so I can't forget them.

MudGuard