my monitor has BNC inputs also, but i have yet to be able to justify the cost of a converter cable just on the hunch that it might help..although i do know at least ONE person who claims there's a quality difference...
truth is, i instead see having two inputs as a cool way to use two computers, but run them both through one monitor. (i.e one with a 15 pin and other with a BNC connector)..seems reasonable, and easy way to switch back and forth...but now i'm getting off track!