Won't this just encourage maintainers to sit on issues... just waiting for the bounty to increase? They might even purposefully introduce painful but simple bugs just to push consumers to pay to have them fixed.
Open source operates in a marketplace today (just not an economic one) so market dynamics would flush this out imo. Intentional bugs, stale development etc would lead to user churn and/or forks. I'd argue faster within OSS where users can see how things are handled.
Having said that, I certainly think the risk of it and fraud more in general increases once an economic marketplace is formed. However, I think those problems are then "good" problems to have and easy ones to fix, e.g flagging repositories with unusually high ratio of repeat pledges, velocity of bugs etc.