Basically class variations and multiclassing is another place WotC has no idea what the hell they're talking about.
Indeed.
But as more of a TO thought, if humored most of Dragon's Variants are subjective to the concept. Not Substitution levels that WotC uses mind you, but Class Variants. Something Dragon readily uses (in fact, most of UA's content is Dragon material edited and updated).
Picture a Martial Monk 2 / Holy Sidewinder Monk 3 / Overwhelming Attack or Passive Way Chaos Monk 2 / Dark Moon Disciple Wild Monk 8 / Major Bloodline 3. ECL 18 but [a combined total of] 27 virtual levels of "Monk" progression. That's 50% more Monk per Monk. :D
It doesn't actually obtain say 2d10 Unarmed Strike since the multiple progressions don't stack with each other. But it's a cool concept.