If we think it's a valuable thing to warn about sibling artifacts used for dependencies, that assumes those dependencies might have changed, which might change the compile or test results of the project currently being built. These are relatively easy to see, since they will likely stop the build. They may not be simple to debug without a message like this, but fundamentally this is a slightly less dangerous state of affairs than the following:
If a plugin has been updated yet isn't used for the current build, the old copy may inject very subtle errors into the final project artifact, which could actually push off the discovery of this error to some point outside the build...even to the production runtime, if integration tests aren't run on that artifact. Same goes for build extensions.
If we dial down this message for plugins, there should be no problem doing so for dependencies. Maybe a better solution would be to find a way to avoid using plugin/extension artifacts from projects in the reactor, that cannot be found in the reactor...I'm not sure how to do this for extensions, but for plugins (that don't use the extensions == true flag) we might use a just-in-time approach to loading the plugin artifact itself...
I'll turn down this error message to debug log-level for now, but open a new issue for later to review the plugin/extension problem outlined above.