Echoing others: There's nothing wrong with taking contributions to a copyleft project under compatible permissive license terms.

That said, CLAs usually include protections that permissive licenses don't. For example, CLAs often require the contributor to guarantee they have the rights to license their contributions, and didn't lift code from someone else without proper permission. Anecdotally, CLAs are also just better licenses, in terms of legal implementation. Especially when it comes to patent rights, though no license can completely eliminate patent risk.