[jkaplowitz]

Widely used CLAs vary on the copyright assignment issue. The FSF requires one, but for example the pattern originated by the Apache Software Foundation and also used by many corporate open source sponsors including Google does not. It does, however, grant them permission to use and relicense/redistribute your contribution outside the bounds of the Apache license that other recipients get.

The copyright assignment does make GPL enforcement easier by having unified copyright, since courts might require that the plaintiffs in a case own rights to a certain percentage of the work or the specific bits being infringed. But there are certainly tradeoffs involved and not all GPL-based CLAs do this.