The 6602 can take an external timebase up to 80 MHz (according to the spec, it's actually a bit higher than this in reality).
The newer counter products do not have this capability (e.g. the 6612 has the same 25 MHz bandwidth limit as most of the multifunction products).
So whenever an application comes up (e.g.) where the user wants to count a fast external signal, the only reasonable option from NI is the 6602. This is fine for now, but in the future it would be nice to have some higher bandwidth counter options (unless you plan on selling the 6602 forever... hopefully it doesn't get too hard to come across computers with spare PCI slots though).