Wouldn't requiring everyone to register weed out a lot of flaky people? What is the "benefit" of allowing people to ask questions without registering first?
The problem is that requiring everyone to register also weeds out a lot of potentially great contributors, too.
It's a cost/benefit analysis: how difficult is it to weed out the truly bad things while allowing the great stuff to happen with low friction?
I found in my years of blogging at Coding Horror that the value of one person happening by with some gold nugget of absolutely the right information you need vastly outweighs the cost of ongoing moderation of anonymous posts.
This is particularly true on Stack Overflow where the cost of moderation is amortized across many thousands of trusted users via the reputation system.
If you get this right, it is an enormous competitive advantage. The value of some guy (or gal) somewhere happening across your page and knowing exactly the right bit of information -- and having zero friction to typing it in -- is something that I believe deeply in.