This letter is rather old, but obviously interesting. Full text: click here
The Congressman can't really say "bugs in free software are fewer", because no-one knows. I can look at Mozilla's bugzilla and know there are X number of known bugs in say Firefox or Chatzilla, but I've no way of knowing if this is more or less than in handcuffware equivalents like Internet Explorer or mIRC, _because they are closed-source_. They can't be fully inspected for bugs, so who knows how many there are?
The next few sentences after the one you post are very interesting, the Congressman makes a similar point about security:
"What is impossible to prove is that proprietary software is more secure than free, without the public and open inspection of the scientific community and users in general. This demonstration is impossible because the model of proprietary software itself prevents this analysis, so that any guarantee of security is based only on promises of good intentions (biased, by any reckoning) made by the producer itself, or its contractors."