This was a popular notion a decade or two ago, but it holds on because a lot of kits, LHBS's, and books still talk about secondary. To make it worse, 'advanced' equipment kits are the ones sold with a secondary fermenter. So new homebrewers are taught early on that they need secondary fermentation without the teachers thinking critically about it. It took a while to convince my wife that this was OK.
Beer will clear up perfectly fine in a primary fermenter. The yeast/trub will form a compact sediment at the bottom, and unless you handle it roughly you can rack it off that sediment easily. A week or two in contact with that sediment will help make sure that the beer is fully attenuated (won't wind up overly sweet) and that 'green' beer flavors produced by the yeast during fermentation will get reabsorbed by the yeast so the beer tastes fresh.
Overly malty beer with bits of diacetyle (buttery) and acetylaldehyde (green apple) - these are probably the key flavors in what people used to call homebrew flavor or twang. These are also problems made worse by using secondary fermenters.