I'll try,

imagine a road like this: http://lh6.ggpht.com/_gt_enTZIj1g/TMq0zFITxUI/AAAAAAAADo8/Rb...

(not sweden but Netherlands but it works for the example.)

Overtaking is not allowed (except for farm traffic), in a 2+1 situation there would be three lanes all the time, alternating between directions. This means that there will always be a legal spot to overtake within a few minutes. (usually the climbing direction would be two lanes so you can overtake slow trucks e.d.) Therefore you don't have to overtake a slow driver by using the oncoming lane.

By a 'standard 3-lane highway', I assume you mean three lanes each way, these roads have nowhere near enough traffic on them to explain all that extra tarmac.