so, dave hit the nail on the head - it's not a catch-all solution. I've managed to fix it up now, it was just a case of marquee selecting over verts and merging if there were more than one.
at last now i know why i kept ending up with these rounded corners - - i was doing the whole drag select/delete but only really looking at one area, not realising it was messing up in others. I can't see why maya couldn't preserve quads in cases like that... it seems a pretty simple rule. but then, autodesk don't do simple do they! =0
well it may not be the solution I wanted, but at least I've gained some understanding. thanks all!!
...select the corner verts first...then shift select..and this will highlight all of the verts you want...then merge....or delete....
good tip bullet -- thanks 
Last edited by se7enhedd; 26-07-2011 at 08:59 AM.