I don't think writing extensions to the language is necessarily bad, nor is that necessarily limited to a single developer goofing around with the code. Let's put it this way: if your program needs to do a bit of linear algebra, would you rather have an embedded language for that, or have a bunch of classes and functions flying around that perform those operations? There is a certain threshold beyond which an extension to the language makes code more maintainable, not less, even when a new member is added to the team.
I think the problem is that people assume that extending the language means that everyone writes their own "for loop." Lisp programmers generally have enough taste not to do that, and if they do it's something more like writing the "iterate" package, which you can control with team policy.
Generally, though, where macros come in is in defining things that would otherwise involve a lot of boiler plate code. E.g. SBCL uses a DSL built on macros to make it easier to write reuasble backends.
