Open Source licenses deal with the question if the license text has to be supplied with the product in paper form or as a file, or whether it is sufficient to specify a URL differently. Most licenses, like the GPL, require that the license text is supplied together with the product. The Landgericht München I (Regional Court Munich), has explicitly deemed this as necessary in a judgment (http://www.ifross.de/Fremdartikel/LGMuenchenUrteil.pdf).
Also irrespective of the license terms requiring the inclusion of license texts in the product, the use of URLs is not recommended in contracts. Contracts are often created for long-term use, so that amendments to a URL can lead to a loss of relevant parts of the contract. In addition, the legal relevance of the URL is greatly reduced since it has to be proven which text was located under the URL at the time when the contract was concluded. This will often lead to practical problems as well as to easy manipulation.