From personal experience, I've benefited way more from that than I've been hurt by it. Whether that's true for all people I don't know.

Of course, you're not supposed to do it other than very carefully and it has caused me problems when some plugin has made a mistake in a way that worked for the original author but not for me, but because of the openness of the system, it's fairly easy to "overwrite it back" automatically when the plugin is loaded – and submit a patch back upstream.

So maybe this sort of system works only when a large portion of the users are hackers, and making Emacs more popular is not a good idea!