Have some XFCP classes that really ought to use traits, but I don't know of any current forum software that requires PHP 5.4 as a minimum.
Also, the fact that XenForo caches the classname after resolving means we can't use different classes for different situations as I initially thought. Specifically I'm talking about the BB-Code formatter. We wanted to extend the formatter directly since VaultWiki has a lot more output formats than standard, plain-text, and WYSIWYG. But we also didn't want to destroy any existing XenForo add-ons by breaking the resolveDynamicClass chain (we want custom BB-Codes to continue working). Instead we have to use one child class that uses "sideways" inheritance which is always really confusing for someone trying to read the source code.
While it would be easy to clear the classCache in XenForo_Application, we can't actually rewrite the resolveDynamicClass method to avoid classes being defined twice. And while it's perfectly feasible to rewrite a portion of resolveDynamicClass in our own code so that our class extends last and doesn't have the redefinition problem, this ends up limiting other developers who may want to extend on top of our class.
So providing public access to certain normally protected things (_tags, for one), then extending out sideways seems to be the most viable option.