I wasn't able to make the event at Harvard about non-competition agreements recently due to my second kid being born, but wanted to share Scott Kirsner's coverage of the event:
Harvard prof. Lee Fleming said that people and ideas move from states that enforce non-competes to states that don’t (think California.) His research has found that non-competes squelch employee mobility by about 20 percent, and 30 percent for experts in a given field. Fleming asked whether non-competes might stifle the reallocation of the best people to the best business opportunities.
A great example of this effect just came to my attention. It turns out that Turbine has been trying to introduce new non-competes. Now, I haven't actually seen the agreements but apparently they have some clauses saying you are not to work for a gaming company within 100 miles (or something like that). Many of the professions in game development are industry specific (level designer anyone?) which essentially means that once an employee has worked at Turbine he is almost guaranteed to move away from Boston for his next job.
This is particularly ridiculous in Turbine's case, since just recently an employee left for Harmonix then thought better of it after a few months and returned to Turbine. With their new non-compete agreement this would never happen, as the person would have likely left Boston altogether and simply taken a different job wherever they had moved. Turbine is poisoning their own pond out of fear instead of building the company from strength.