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.
And also in some ventures it might be pretty hard to litigate when everything is done behind closed doors. How would you know if a rival trading firm is using an algorithm influenced by yours or not?
But, maybe to your point, just because it's illegal doesn't mean it's easy to litigate. Much of the legal system is specifically to avoid litigation. Apropos to this discussion, even if a contract isn't enforceable, it only has to be perceived as having teeth to give it value. It's like when a baby elephant is tied to a stake and it grows to an adult still thinking that stake prevents it from leaving. All it takes is for an employee to think a non-compete has merit to keep them in place.