It is possible that the lack of useful contributions is due, in part, to a CLA being required.
An alternative would be to instead allow the author to keep copyright but require dual licensing (e.g. both GPL and MIT||BSD) for patches to be accepted into the main branch. The patch meets the GPL requirements, and the company can use the other license when distributing the patch with their own proprietary releases. Does that occur?
Is your main objection to CLAs copyright assignment, or that you prefer licensing to be pure GPL, or something else?
Edit: Rack say “To accept a contribution, all authors of the contribution need to: declare the patch under the CC0 license, or complete a copyright reassignment form, or perform the work under a paid agreement.”
The complication is that, if you received the source under the gpl, then you must release your patch under the gpl, too. But I think this can be worked around by the company licensing the source to you under a different license, that allows only submitting patches.
So basically, you effectively submit the same patch twice: once under the gpl, based on the public gpl'd code, and a second time under a proprietary license, based on the same code, offered to you under a proprietary license that exclusively allows sending patches.
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.