This is something ministers in churches should do. I have attended churches where I subsequently left for some reason (other than moving from the immediate area). More times than not, I do not return. I am not an advocate of church hopping, but situations come up that have lead me to leave a church.
There have been times, though, where I have reconsidered my original decision, and knowing the pastor would welcome me back would help my decision on whether to return to the congregation. However, I cannot recall a minister letting me know that I am welcome back should I change my mind in the future (I do recall one lay elder letting me know I was always welcome back). I have had pastors ask me if I would reconsider staying with the church. The problem with this approach is that it feels like a door is shut should I decline to stay. However, if the pastor added that I am always welcome back, this helps me understand that the door is open for me to return.
Returning to a congregation can be somewhat embarrassing because it can appear that the original decision to leave was in error. This is not necessarily so. In fact, leaving can help an individual or family realize how well they fit into their former church.
It is much like applying for a job. I once knew a Dean of a college who encouraged his faculty to apply for other faculty positions should they find one they desired. He said such applicants often find by going through the interview process that they like their present faculty position better, and such faculty find they appreciate the support they received from the Dean during the process. In a similar way, a pastor who verbally lets individuals and families know they are always welcome to return shows that he respects them.
I do wonder if the reason pastors have not said such welcoming words is because they feel rejected by the members leaving. In my case, as a professor, I let my students know that I do not take their decision personally, and that they must do what is best for them in their academic career. Likewise, a pastor letting individuals and families know they are always welcome back lets them know he is not hurt by their decision. Plus, by doing so, should he come to doubt himself in his ministry, the pastor can assure himself that he has done what he can to make such people feel welcome.
Finally, I recently listened to a talk (podcast) by Reverend Alistair Begg entitled, "The Dangers and Delights of Long Term Ministry in One Place," given at The Gospel Coalition 2013 National Conference (April 09), where he said one of the delights in a long ministry in one place is to see people return to his church. He said some members have the "Little Bo Peep Syndrome" (leave them alone and they'll come home, dragging their tails behind them) where members leave because of something they do not like about the church (youth ministry, politics, etc.). He noted that a minister should let such members know he hopes they will come back. Furthermore, should a minister stay at a church long enough, he said such ministers will find many of these members do return believing things have gotten better. He recommends the minister let these people know he is sincerely glad they have returned and that he is privileged to preach the Word of God to them. I think this is an excellent attitude to have.