how many times do people request someones website? , if its not in their signature. I think its a good idea to have licensed url under avatar and it will help with some of the nulled sites too.
Also not everyone wants their URL to be known. We have to respect that wish rather than forcibly publishing it for all to see.
Therefore as it is the user's personal choice, there are already ways to do this. There's a home page URL section. There's also the About section and Signature.