What do you think? In the competitive market we are all in today we all know the demand for top flight talent will be fierce. No doubt. Under certain market conditions are you willing to bend the rules or are you steadfast in your policy not to rehire? I think it deserves a healthy debate. If the employee was laid off —that's a different story. Let's look at a few pro's and con's of bringing back a former employee:
Productivity (+) - Re-hiring a former employee almost assures that they will hit the ground running and that has inherent organizational value. The former employee will likely require little to no ramp time before they are highly productive and begin making an impact. That's a good thing.
Organizational Culture (+) They already know the organization and how you operate which should prove to be very positive. Knowing how to get things done and who to go to in order to make an immediate impact and avoid bottlenecks has real value. It would likely take a "newbie" much longer to understand how things are done organizationally.
Impact on Existing Staff (-) This could be tricky depending on how they left and what your team thinks about the person in question. After all the staff you have in place could think that since they never left the organization they are much more loyal and deserve some credit for sticking it out. Re-hiring defectors almost condones that leaving is OK and the door will always be "open". You and I know that's not the case for all employees.
Implied Knowledge (-) This person could assume that things are business as usual and resort back to the way things were before they left and convey a "know-it-all" attitude to the rest of your team. Not good. Most organizations are dynamic and always changing. If you are still doing everything the way you used to - well that's a whole new issue. They could undermine a former peer who is now a manager - also a big issue.
Today's market forces all of us to think differently about talent acquisition if we plan to remain competitive. I don't have the answer on whether you should or should not re-hire. However - you need to figure out the answer to this questions - soon.