I'm certainly not trying to put you on the defensive here, so if I've done that, my apologies.
My point was that, after having played several pure-class characters, I didn't enjoy playing them as much because of their inherent limitations, and, in most cases, that their strong dependencies upon other classes are a huge limitation in the all-too-frequent occasions where characters of the dependent classes simply aren't available.
Haven't you ever been playing along and think "I wish my character could do that!"? In many cases it isn't about synergies, but about what you want your character to be able to do. I mean, just because you can climb a mountain doesn't mean that you can't *also* dive the ocean. My main character is a very complicated multi-classed arrangement that's often quite the challenge to play. But I find that provides me with more personal enjoyment of the game.
My main character currently has two levels of Rogue, and he can find and disable most traps in the game, even on Elite, and open most locks in the game, even on Elite; he has five levels of Ranger, so he can dual wield most weapons in the game and is quite effective with all manner of ranged weapons, not to mention the fact that he can always heal himself and both heal and repair others in the frequent absence of a Cleric; and he has seven levels of Wizard, meaning that he can cast every arcane spell in the game up through fourth level spells at the moment. To pull this off requires a great depth of understanding for each class's requirements, but also an understanding of how portions of each class's abilities *can* be combined effectively, even if they don't *appear* to be synergistic.
In the end, multi-classing gives *my* character the abilities *I* want him to have, and I enjoy that aspect of the game quite a bit. The drawback is that most players who don't multi-class don't understand it, and aren't willing to open their minds to the associated potential. And, that's why the OP asserted that multi-class characters are relegated to the status of second-class citizens.
Sometimes it's not for others to understand, but rather, to simply accept.