Sustaining foci are don't have to be in contact with the magician that has bonded them, they just have to be in contact with the target of the spell that they are sustaining. This can lead to some very very nasty situations where a magician gives a sustaining sustaining focus to an unsuspecting person then casts a malicious spell into it.
It can also be used by less scrupolous magicians to make a quick profit.

Lets say a rich corp exec is infatuated by someone who doesn't return his or her affection. There two classic solutions, bullying and persona-fix BTLs are messy and dangerous. A sustained love spell, on the other hand, is much more subtle and doesn't change other aspects of the target's personality. All this exec would have to do is drop a ring that's really a sustaining focus. The object of his desires picks up the ring and the mage casts the spell. Then, the exec simply insists that his new true love keep the ring and imediatly gives her another piece of jewelry, which really a sustaining focus for control thoughts (Never take off these two pieces of jewelry), thus ensuring eternal love without the personality damage that comes with more traditional forms of brainwashing.

Of course, the mage would be a sitting duck for if anyone capible of Astral Tracking or Ritial Sorcery gets one of his foci and using magic to brainwash is certainly highly illegal in most areas.
The tequnique can be useful in Shadowruns, as well. When covertness is required, having a lover on the inside is a great way to gain intelligence. Of course, if the orginization has any astral security, this won't work.
It can also be used by Johnsons who want to eliminate all links to himself. He just pays the runners with certified credsticks that is also a sustaining focus for the nasty spell of his choice. Pertify really works well. The Johnson can just track down his foci runners one-by-one and smash the petrified runners to into itty-bitty bits with a sledgehammer.
Confusion is less evil but just has fun.

The trouble with sustaining foci for mind-control spells is that the mage has to keep concentrating on what he wants the target to do - he can't give some instructions and then wander off. Keeping such a foci active full-time would really not be practical. And what mage in his right mind would do that with a magical item that can be traced back to him? I don't see sustaining foci being used that often, but they could still be used for a number of nasty things - but more likely with prisoners and the like, since sustaining foci are too much of a vulnerability to the casting mage to simply let the person with the sustaining focus walk off with it.

You know, a sustaining focus, combined with a spell like chaos, hot potato, or agony would be a really nasty way to torture someone without leaving any marks.

I don't think the mage has to concentrate on controll thoughts as long as the thought being transmitted doesn't change in any way... it should just keep the same thought transmition (keep the ring on). Personally, I'd use influence (don't take off your jewelry).

JaronK

Control Thoughts into a Sustaining Focus would keep the last commanded thought sustained. No new thoughts. As a GM I'd even consider the "Caster not present" since you're no longer sustaining the spell, and allow a measely 1-success to overcome the spell permanently, each time the controlled thought repeats itself.

Confusion on the other hand, I do think it great fun and a great idea to do that with a sustaining focus. I even did the area-effect quickened on a bar once to keep people from listening in (thought he GM decided people would not believe it part of their drunken stupor since not enough were really drunk, but it did help vacate the premises).