Cap 56
The first responsibility of ATC is to separate the ACFT.
The second is to do this as efficiently as possible.
Visual approaches are part of that goal, hence they can suggest it.
"Safe, Orderly, Expeditious" in that order. Whilst visual approaches might be expeditious, there is potential for disorder and breaches of safety if a pilot is forced into a visual approach he isn't prepared for. It is one thing to suggest a visual approach. It is another thing to clear an aircraft for a visual approach that wasn't requested. These destabilised approaches and associated go-arounds that the others are talking about will take away all your efficiency and then some. Of course, if they miss the airport completely and bend the aircraft landing on a nearby highway, or smacking into a hill at night, there will be a lot of paperwork to say the least.
There are some things that ATC can suggest to pilots, but there are some things that are best left for pilots to request.
Obstacle clearance remains the responsability of the pilot even when radar vectored.
I'm curious to know how you expect a pilot to maintain terrain clearance when the controller is positioning his aircraft all over the sky. Where I work, I am responsible for terrain clearance when I issue a descent clearance based on the terrain map, or when I commence vectoring. In either instance, I must make it clear to the pilot when I am handing that responsibility back to him. Having said that, if I had you on a heading that you thought was pointing you directly at a big lump of rock, I wouldn't be offended if you asked where I was planning to vector you and/or could I confirm Radar Terrain.