CLA's basically do one thing: Protect the company using them from being sued by a contributor, or from being sued if a contributor copy pastes copyrighted code into the project. They're there to protect corporations that have outside contributors to projects they build and sell, typically. Sometimes, they're fine, like if you signed one to work on some obscure Linux driver for an obscure hardware item: the hardware company might want an outsider to write it, but still need legal protections since the person basically wandered in off the street from their legal departments view.

Sometimes, CLAs are bad. Oracle loves CLAs. Sun Microsystems loved CLAs. Pretty sure you have to sign one to contribute to OpenJDK now...

Open source licenses can be just awful too. AGPL, for example. CLAs are not an immediate "no," but they should make you ask "Why am I signing this?" If you can't come up with a good reason, fine, don't sign it. But don't for a second think you'll get code into a project run by Oracle without signing one.