Expedition to the Barrier Peaks (and, later, Blackmoor) to soften my stance on this question, but, even then, I was still instinctively opposed to the notion, despite ample evidence that the older generation of gamers, to whom I looked up, had no such qualms about it.
Consequently, I was equally baffled by both the sections of the Dungeon Masters Guide that included guidelines for converting Gamma World and Boot Hill characters to AD&D (and vice versa) and by articles like "Modern Monsters" in issue #57 (January 1982) of Dragon. I can't be certain, but I think this was the first article I ever read written by Ed Greenwood and I didn't think much of it at the time. Stats for cars and bazookas? Advice and guidelines regarding the functioning of magic in modern, technological worlds? Why? What was the point?
Now, there was no question that Greenwood's article was well presented and made ready use of ideas and suggestions from earlier articles (like James Ward's "Monty and the German High Command," for example). Heck, the article even had footnotes, citing not just previous Dragon material but a host of classic fantasy and SF literature where magic and technology contended. But none of that moved me and I found myself unable -- or unwilling -- to shake off the staid notion that six-guns and sorcery should never appear side by side. It's only been in recent years that I've abandoned my adolescent scruples regarding genre mixing and happily included robots and laser pistols in my fantasy without regret. Live and learn!