A common problem with large projects starts with adding new programmers. What happens is that the new coder bypasses the existing work and jumps right into adding a clump of code. Frequently, both to make their mark but also because of past experiences, they choose to construct the code in a brand new, unrelated way. Often, they'll justify this by saying that the existing code or conventions are bad. That's were all of the trouble starts.
In terms of good or bad, most coding styles are neither. They're really a collect of arbitrary conventions and trade-offs. There are, of course, bad things one can do with code, but that's rarely the issue. In fact, if one had to choose between following a bad coding practice, or going in a new direction, following the existing practice is the superiour choice. The reason for this is that it is far easier to find and fix 20 instances of bad code, then it is to find 20 different ways of doing the same thing. The latter is highly subject to missing code and bug-causing side effects, while the former is simple refactoring.
When a programmer breaks from the team, the work they contribute may or may not be better, but it definitely kicks up the complexity by a notch or two. It requires more testing and it hurts the ability to make enhancements, essentially locking in any existing problems. And as I said earlier, it is often no better or worse than what's already there.
It's ironic to hear programmers justify their deviations as if they were improving things, when experience shows that they are just degenerating the existing work. Once enough of this happens to a big code base it becomes so disorganized that it actually becomes cheaper to completely start over again, tossing out all of that previous effort. Overall the destruction out-weights any contributions.
That's why it is critical to get all of the programmers on the same 'page'. It is a sometimes-painful necessity to allow any large development project to achieve its full potential and it is one of the core ingrediants necessary to forming a 'highly effective team'. The latter being one of the keys to building great systems; in that dysfunctional teams always craft dysfunctional software.
With all of that in mind, there are a couple of 'rules' that are essential for any software project if it wants to be sucessful. The first is that all of the original programmers for a new project need to come together and spend some time to lay out a thorough and comprehensive set of coding conventions. These should cover absolutely 'everything', and everyone should buy into them before any coding begins. This isn't as difficult as it sounds, since most brand new projects start with very small tiger teams.
The next rule is that any new programmer has to follow the existing conventions. If they don't they should be removed from the team before they cause any significant problems. It should be non-negotiable.
Now of course, this is predicated on the idea that the original conventions are actually good, but often they are not. To deal with this, a third rule is needed. It is simple idea that anyone at anytime can suggest a change to the existing conventions. For this to come into effect two things are necessary. First, the remaining programmers all need to buy into the change, and second the programmer requesting the change must go back to 'all' existing instances in the code and update them. The first constraint insures that the new change really is better than what is there, while the second one sets a cost for making the change and also insures that everything in the code base remains consistent afterwards (it also teaches people to read code and to refactor).
If teams follow the above rigorously, what happens is that the code base converges onto something organized and well-thoughtout. It does add some overhead, but a trivial amount relative to a messy disorganized ball of mud. And of course it means that the development project never ends for avoidable internal reasons.