I can understand using the middle lane for a few overtakes / past slip roads etc. but when they refuse to move back in, it simply makes a 3 lane motorway into a dual carrigeway.
For the most part I think there's 2 schools of driver that does this:
1. The lazy driver that knows better but can't be assed to keep swapping lanes (let me generalize: XJ Jag / Lexus GS / Audi A8 / any comfortable cruisers)
2. The incompetent driver that is afraid of moving lanes because they know their sh*te at checking blind spots etc. and its a big deal when they need to make a junction.
(car type: ermm, something cr@p and built in the Far East)
Overall, they are much better at this on the continent. In France / Germany, the overtaking lane, really does mean that, and most people will return back into the driving lane after an overtake.
If someone has their indicator on, people move over... just great that... none of this dangerous slamming brakes on we get here (classic YT video there ^^^).