As a general practice, it's liable to devalue volunteer contributions. Money poisons the good will of a free exchange in the mind of a volunteer. Strict remuneration policies are an important part of making volunteer labor feel fair, free, and balanced. I suspect there would be room for targeted bounties on specific articles, but it would nevertheless irrevocably alter the emotional economy of contribution.
A 3rd party paying editors introduces a host of other concerns, but is probably not as harmful to the emotional economy. But I'm sure Wikimedia's community team has thoroughly discussed this topic.