The more traditional Z-Axis is much more precise than using the Rigid router’s base. The router base has a fair amount of backlash in it and I’ve seen it drift as much as 3/16" from what it should be. Using a bungee to preload helps, but its still not nearly as accurate as it would be with a lead screw, or even better, a ball screw.
It also has the added bonus of giving the operator the choice of different routers/spindles. 