I think that the way OD&D, AD&D, and BECMI were designed, level disparity is perfectly fine. When I played, most people had multiple characters of various level, so that they could pick a character appropriate for the adventure. In 2E, we had more story based games, but due to the prevalence of Raise Dead by NPCs (and eventually PCs), we very rarely had a situation where a player ended up significantly behind the party. New players started with the total XP of the lowest character; an affectation that carried on to 3E.
Level disparity was built into 3E, but in a bad way. Level loss due to death meant that there was a built in doom spiral, where the weakest character would die more and more often. I've seen players leave games after their 3rd death in a row, knowing they were just going to die again. The XP system was supposed to give more to the lower level characters, allowing them a chance to catch up again, but IME it never came close to making up for one lost level worth of XP, let alone multiple levels.
I have not seen level disparity during 4E or 5E, and I strongly suspect it wouldn't work very well for either edition.