I will dig in my heels on this one, and provide a few more counterexamples.
@
lonesomesteve 's Frek has the stem drilled for a bell. I believe it is also an SR Custom. He rides a
lot and is still alive as far as I know.
Brian Chapman and Peter Weigle have also done it, if their Flickr pages are to be believed. And according to one issue of Bicycle Quarterly, the author asked Nitto about drilling a stem for a cable stop, and was told that was fine. By Nitto!!
A couple of my friends have drilled and tapped stems also. I have never seen or heard of one that has failed due to a drilled and tapped hole.