But I do understand it. When I first entered the hobby, I tried to get into the various authors and stories the older guys said were "important" for me to read, like Burroughs, DeCamp, Pratt, and so forth. Try as I did, though, I can't deny that, back then, I found a lot of this stuff boring, especially when compared to the "modern" fantasy books that were all the rage back then, like Terry Brooks and David Eddings. And those books did not include modern day characters traveling to other worlds (or, to the future, in the case of Brooks) and interacting with all their fantasy creatures and situations. They were serious fantasy, after all.
Of course, had I bothered to look at Appendix N, I might have noticed the large number included in it who wrote stories that involved a 20th century man traveling into a fantasy world:
- Poul Anderson's Three Hearts and Three Lions
- Edgar Rice Burroughs's Barsoom, Amtor, and Pellucidar books
- DeCamp and Pratt's Harold Shea books
- P.J. Farmer's World of Tiers series
- Fritz Leiber's Fafhrd and the Gray Mouser do the opposite, being fantasy characters who travel to our Earth, albeit in the past
- H.P. Lovecraft's Dreamlands stories
- Abraham Merritt's Dwellers in the Mirage
- Michael Moorcock's Elric of Melniboné frequently travels to other worlds, including our own.
Looking back, I think what has happened is that, as the fantasy genre has changed over the years, it's opted strongly for "self-contained" worlds that are separated from our own. Although there are exceptions -- Donaldson's Thomas Covenant books, for example -- they're mostly outliers. Instead, when people think of what a fantasy novel should be, they tend to think of The Lord of the Rings as a model (even though Tolkien intended Middle-earth to be the mythic past of this world) rather than something like A Princess of Mars or "The Roaring Trumpet." Somehow, what had been a mainstay of fantasy for the better part of this century has been reduced to a curiosity, particularly among gamers who aren't familiar with the pulp fantasy literature from which the early hobby took inspiration.
All of this is simply a long-winded way of saying that, far from seeing any problems with the introduction of a 20th century man into my OD&D campaign, I see his presence as every bit as natural as the presence of a spaceship in the World of Greyhawk. Of course, lots of gamers still have problems with that too, but at least I'm in good company.