Between 2000-2005, computer company SUN Microsystems developed a number of major improvements (zones, ZFS, dtrace) to Solaris, which was their version of the unix operating system. Then in 2005, SUN released all Solaris code publicly through a CDDL license along with 1600 patents. The CDDL license was GPL incompatible, and therefore linux authors could not copy Solaris code and claim as their own.
The linux authors potentially had the option of rewriting zones, ZFS and dtrace for linux, but they actually did not because of the patents. If linux incorporated any Solaris idea into mainline linux, the Linux Foundation and related businesses could face patent lawsuit from SUN or Oracle, the successor of SUN.
Long story short, (legitimate) software patents do have the power to deter GPL and other ‘open source’ code rewrite options.