Does patent rhyme with open source? Think about it this way - the original GNU Public License is a form of managing intellectual property based on copyright protection. A restriction _is_ being placed on the intellectual property licensed with GPL, specifically that modifications cannot be held proprietary. If open source did not place any restrictions on intellectual property, then Public Domain would have been sufficient and there would be no need for the GNU Public License. That the restriction is a permissive one (aimed at increasing the availability of the source code) does not affect the fact that open source licensing requires being able to legally restrict the kinds of things that can be done with the intellectual property.
So, is copyright a sufficient mechanism to propagate open source? If a closed-source company encounters a particular piece of code that it likes, why couldn't it simply take the code, make some nominal change so as to avoid copyright infringement, and then incorporate it in to its proprietary products? This has probably happened on a number of occasions already.
If the source code embodies a truly novel or innovative feature, then the copyright basis of current open source licenses may be too weak. In this case, adding a patent (provided the feature is patentable) as an adjunct to the open source license can help to minimize the temptation of closed source companies to simply reverse engineer the feature and incorporate it in to their product. Of course, it should be agreed that the patent should not be used to restrict the open source propagation of the intellectual property (which is specified in more recent versions of the GPL, I think).