If you're worried about the top warping, you could build a torsion box top. It would take a lot of weight to make that kind of top move. The inserts are really handy. Depending on how you're set up for bit changing and lift, removing the insert with router attached is just too easy. With the newer routers, heights can be adjusted from the top, also.
I dedicated a fixed base for my table and it makes life very easy. If I want to pull the motor to do some plunge base work it is a matter of reaching into the cabinet, loosening the clamp, and unplugging the cord. I only have one router so it was an easy choice to do it with an insert and dedicated base.
Sawdust is not dirt