why open source projects (in particular) need a bugmaster

Of course, bug tracking when you have paid QA in a proprietary situation and bug tracking when you have volunteers in an open project are two different beasts. Open source projects need bugmasters most obviously and most badly; besides the reasons listed above, open source projects have:

Massive numbers of external testers that can quickly make a bugzilla useless. Massive open beta is one of free software's biggest strengths, but it is useless for large projects without organization- hackers just can't keep up. Too many projects have thrown up their hands and said 'I can't keep up without more hackers' when the real solution is not to throw more hackers at bug system maintenance, but to throw an expert at bug system maintenance and let the same number of hackers hack. This is less of a problem in proprietary projects, where the ratio of QA to developers is likely to be small (a handful of developers per QA person), instead of the (effectively) hundreds to one in most successful free software projects.
A good bugmaster can help create effective testing and QA communities- strong, directed leadership is always helpful in creating communities, and a good bugmaster can help create this. In the best cases, open source communities do happen almost by magic, but having someone who feels responsible for helping create such a community can help a lot, especially in QA, which is not typically a 'glamour' job in open source and hence isn't as likely to create community as hacking or art.