Presumably this was the official TSR line (though I'd be interested to hear if there were contradictory statements). But is this what veteran players really assumed back in the day? A wall between Basic D&D and AD&D, with nothing meant to be used interchangeably?
I tend to see them as two attempts at the same basic formula. There are differences between BX and BE that are larger than either to AD&D (and I've always said that the biggest edition change in D&D history takes place
within* oD&D)
*That's actually a huge spanner in the works of these discussions. Many versions of the game have, within each edition, radical change in the rules, structure, settings, feel, tone, and all the other distinguishing characteristics we might use.
In 1983, me and my friends had one or two sessions where the 'big kids' (10-11 year olds) let us play in their hybird (Holmes/Moldvay-Cook/AD&D/some oD&D supplements and adventures) game. We then started our own group, and depending on when each of us saved enough allowance, had a birthday, or begged our folks to get us a set, got BX, first printing BE (which had some stuff that was later changed), or 2nd+ printing BE. We noticed right away that they were different, but (given that getting the same books for everyone was a non-starter) made the whole thing work.
They were not totally interchangeable, but you could adjust for them on the fly as a 10 year old.
In the end, it is a matter of perspective and opinion.
That's about where I stand too, if the youngest (we did it at 8) of gamers could recognize them as having differences, but also be able to make them work, then their status as wholly different games is pretty ambiguous.
They also maintained it because it sold EXTREMELY well worldwide.
Yep. A lot of us in the USA did not realize that basic/classic outsold AD&D abroad. Given that, you sure would have thought that they would have put more effort into it here (I guess all the gazetteers and stuff would be a lot of effort for something they only had to show they were still publishing).
For those that say they used material interchangeably, do you mean that I could play an Elf (as in B/E elf) in an AD&D game? Or your weapons in an AD&D game all did d6 damage? What do you mean by interchangeable?
You took whatever people or the DM wanted from whatever source, and worked out the details. I don't know anyone that played a BE elf in and AD&D game, but then again the elven fighter-magic user is right there (that might actually be a guiding principle: take from a different source if said thing doesn't exist in products you own of the mainly-played source). As for weapons, I don't know anyone who started with BX or BE who didn't use the variable weapon damage rules either, so if you were playing Basic, a sword did 1d8, and if playing AD&D, it did 1d8/1d12).