I usually start a secondary at level 10 on my characters, as getting a secondary really makes leveling easier for some classes (warrior is god-awful until you get a secondary so you're not as dependent on rage). I like to keep them fairly even until ~30 or so, and then I let the secondary fall behind by about 10 levels.
As for not dual classing...no, there aren't players that do that. Or if they did, they wouldn't make it too far, because any class will be gimped without a secondary. The game revolves around the dual-class elite skill system; if you don't have any elite skills, you're really missing out. Some elites are what make certain combos so powerful.
