Reason Acura dropped the Integra name was because they wanted an alphanumeric system for the entire lineup so that people could identify with the Acura name more.
The NSX, RSX and MDX have no numbers like the TL, RL and CL because those three are considered the 'extreme' cars, hence the X (trying to remember what I took in from a similar conversation over at h-a.net). The TL, RL and CL are considered the normal cars of the line up, hence the numeric and alpha names. Wish I could dig up that old thread back there...