[KaVir]

Using existing software as the starting point for a new version is actually a fairly common form of code reuse, and one I've seen to varying degrees in every professional project I've worked on. Even in my own mud, I've reused various standalone snippets that I've written and released in the past (such as my soundex parser, my dynamic description parser, my text justification snippet, etc).

It depends on what they're using. It could be some low-level functionality that can simply be compiled and used in the same way as a library, it could be an LPMud-style driver with the owner developing within in a sandbox...but even if they decide to modify what they've downloaded, unless it's very poorly written and/or documented it should still be faster than writing it all themselves.

A copy of that mud, but that's not what we're discussing. The question here is what codebase is going to give someone a faster development time than writing from scratch. And the answer will obviously depend on what they're trying to do - there is no "silver bullet" codebase that'll fix every problem.