People are using PD-7400's without clips because clips other than size L are virtually unobtainable, and the clips tend to be a little cockeyed from where some people like to have the toes of their shoes. Plus, you don't really need them -- they help you do an on-the-fly flip-up-your-pedal-and-engage maneuver, but on the track that isn't really necessary. The front-of-the-pedal winged bracket that clips the front of the PD-64 cleat really is easier to get into without clips in the way (or at least that's my own feeling).
PD-7400's are mandated by a couple teams (France, for example) and optional for others. The advantage of an SPD-R is still that you don't have to strap your foot down hard just to keep it in the pedal because the SPD-R engagement mechanism does that for you.
By the way, if you haven't tried the new Yoshida or MKS slotted cleats, you should check them out. The MKS are beefier but have a bump in the front to align them on MKS pedals, but the bump tends to sit on top of the winged bracket at the front of 7400's. If you remove the bracket, they work very nicely and are deep. The Yoshida's are not quite as deep and I've seen a couple break after heavy use, but they do come in colors and fit just about any pedal. Both cleats fit Look drillings. If you think PD-64 cleats are hard to find, try SPD-R drilled shoes.
The Exa pedals seem to have lost out to SPD-R's, which don't require that you flip a lever to engage or disengage. The cleats are nice (very similar to the MKS cleats mentioned above but definitely not interchangeable) and Look-pattern, but the pedal is just a bit heavy with all the mechanism and there are simply equivalent solutions available. If you like them, however, they are solid and reliable, and you don't have to find SPD-R drilled shoes. They aren't NJS, by the way, which is part of why you don't see them used on the keirin circuit.