At the end of the interview you tell the employee candidate that they will hear from you. It can be a call or letter asking for a second interview, or letting them know they got the job or that the job was given to someone else. You made a commitment to that person to communicate with them. What does it say about you and the company you work for when you fail to follow through? Think of it this way, would you treat a customer that way? Then why would it be ok to treat any one that way, it still has an impact on how people feel about the company.
How you treat potential employees shows how you and the company deal with people. Taking the time to call helps with positive branding. If some one did not get the job getting a call letting them know that shows that the company follows through on their commitments, and treats people with common courtesy and respect. People are more likely to continue seeking employment with the company. It is easer to find quality employee candidates when people see the company as a place they would like to work at.
By treating every one with common courtesy and dignity you leave your options open. You never know what is going to happen. What if the person you hired does not work out. Do you need to go through all the hassle of interviews again, or do you call back the other top candidates you did not hire and just re-interview them? What if one of the candidates was not right for the job that they were interviewed for but another job opens up that they would be perfect for. If you did not let some one know they did not get the first job that indicates you did not have any respect for them. Why would they want to work for you?
Following-up with employee candidates after an interview is part of the hiring job. If you do not do that part of the job what else are you not doing? Do you do every this way, only doing the part of the job. How reliable and trustworthy are you. Oh, you did not think any one noticed that you did not contact the employee candidates who did not get the job. Well I am sure several of those people took the time to call when they did not hear any thing from you. How professional do you look now?