Getting fingers burnt
A few years back I did a little bit of vectoring on a departure to keep it out of the way of an inbound. Instead of turning pretty much due North immediately, I took the aircraft about 10 miles West before clearing it to resume own navigation direct to the first RP on the departure route. Rather than turning right and going in a straight line to the RP the aircraft almost made a 180 degree turn toward the overhead (and, rather worringly, straight at the next departure). After I sorted the immediate problem out, I queried what the pilot was doing he told me he was turning to intercept the departure track as quickly as possible. He was adamant that not only was this the correct thing to dobut that I should have known that that's what he would do.
I can't say whether this is common practice - I'm not going to fall for it again!! In the same situation these days days I'll either put an aircraft on a heading to where it's supposed to be going and when well out of the way of everything put it back on its own nav or keep it on a heading and transfer it on a heading.
Yes, it's more work. But I still don't understand which bit of direct is so difficult to understand!