Can the controller "be assured of what you've got" when you report "traffic in sight"? Moreover, can the controller be assured that it will stay in sight?
That's not really relevant. A controller who hears a pilot report the preceding aircraft in sight gives separation responsiblity to the pilot making the report, whereas a pilot reporting seeing an aircraft on TCAS does not provide that latitude. Further, I've seen far too many pilots who are satisfied when they see an object on TCAS. Additionally, most operations manuals give mandatory RA reaction to TCAS, taking that discretion from the pilot.
The July 2002 collision between a Tupolev and a B757 over Germany involved two aircraft equipped with TCAS, in radar contact, talking to a controller. A chain of events occured, including several mistakes, which lead to the collision, but the fact remains that even with professional crews operating advanced collision avoidance equipment, getting radar advisories are still not immune. Further, taking evasive action based on what you don't see, be it based on TCAS or based on what the controller tells you, still does not make you immune, or necessarily improve your odds.
http://www.humanfactors.uiuc.edu/Rep.../nuneslaur.pdf