The two main issues I have with them are that firms tend to give them to just about everybody (instead of just to folks working very directly with real IP), and they only pay base salary, not something closer to actual total compensation (often multiples of the base pay).
Having said that, the quant firm is relatively unimportant and not a good reason to prevent a total noncompete law. It's probably better to just ban them then try and make allowances that aren't full of loopholes.
Cry me a river. If knowledge of some particular employees worth so much to the quant firms, then they should pay them not to leave accordingly.
The OP was about how non-competes make sense in an IP-intensive field, like quant finance. The reason is that these contracts help protect the IP by explicitly stating their case. Your comment goes against the very foundation of IP law: creating reasonably fair commercial opportunities. If I can extort you because you hired me and I learned your secrets, I think that pushes the scales beyond "reasonable."
However, this might be confusing different issues. My comment was specific to using NDAs/non-competes to protect trade secrets. This is different from merely using them to prevent poaching by competitors. In cases were there isn't inevitable disclosure, I think it's much less likely that a non-compete would be enforced in court.