Irreplaceable is good. It means that person is a superstar. Keep them happy (and safe!).
One danger I have seen manifest itself when splitting code up into well defined areas is that participation in the code base as a whole drops significantly. You might get a few drive-by bugs between areas, but not the kind of serious review you give your own code.
Also, when one person is "responsible" for an area, one area is almost always the limiting factor (for development speed, quality, etc.).
Finally, having clear areas of responsibility like this mean that the manager of the developers can reinforce bad behavior (i.e. this bug is not MY fault, see, it is in HIS area). Then, everyone sits around and complains instead of someone just pitching in and fixing the bug.
One danger I have seen manifest itself when splitting code up into well defined areas is that participation in the code base as a whole drops significantly. You might get a few drive-by bugs between areas, but not the kind of serious review you give your own code.
Also, when one person is "responsible" for an area, one area is almost always the limiting factor (for development speed, quality, etc.).
Finally, having clear areas of responsibility like this mean that the manager of the developers can reinforce bad behavior (i.e. this bug is not MY fault, see, it is in HIS area). Then, everyone sits around and complains instead of someone just pitching in and fixing the bug.