It should not worsen rigidity of a design, but without it the workbench loses part of functionality.
Options like a lattice from the bar covered with plywood of 12-20 mm do not approach.
To parallelism and quality of the planes of a vice not less strict requirements, than are imposed to the main surface. In process of wear of a sponge it is possible to change, the mechanism needs only periodic service.
One more advantage – height of a workbench is regulated smoothly, and not so difficult mechanism, as in wooden designs.
However true professionals consider that steel in a joiner's workbench – alien material. These traditions leave the sources nailing in those days when handymen collected wooden fellings without uniform.
Besides, ideally flat workbench already in itself is a standard for alignment of components of a product, or system of coordinates at a marking.
The flat surface will quickly wear out, and in operating time constantly there will be hooks of preparations about roughness.
Strong table-top (working surface) from strong breeds of a tree. The oak, a beech or a hornbeam will approach. Density of the top plane has to be higher, than at a tree of potential preparations.
Advantages of a design – many resources are not spent for linear strengthening of legs.
The design of the worm mechanism can be metal, most often and happens. But sponges are surely made of a tree. And the tree has to be soft – not to leave traces on preparations.
Optimum option – type-setting a table from boards 50-70 mm wide. The massive table-top gives to rigidity of all design, and will not cave in under the weight of preparation of any size.