My personal opinion is that donations will not signficantly affect the manpower allocated to tasks. If someone were motivated by money, the donations would be unlikely to cover the opportunity cost of performing the tasks. If someone were not motivated by money, they would complete the task anyway (if it were something they were interested in).

A more effective solution would be to contribute code, rather than money. Of course, the opportunity cost of contributing that code is typically far greater that the sum which would be donated, which brings us back to my first point.