In all fairness, do you ask marketers if they go to networking events to gauge their interest in developing business relationships, or would you count that as a negative, because they're also exposing themselves to being scooped up by your competitors?
Do you ask sales people if they have independent side commissions, or would you consider that as cutting into the time spent chasing the deals your company looks after?
Do you ask strategic account managers if they take the time to visit their clients beyond an employers' requirements, or do you see that as a liability for having them poached as in-house talent?
I get that you want developers who show an intrinsic interest in staying current, because the field often requires training in deeper levels or new technologies. But that's different than avoiding someone who's obstinate about always doing things the old way: no one wants that in any role. The real question I think you're missing is this: is it the employee's job to do this on his/her own time, or should the company be providing the time and resources into it as normal staff development?