Hello - if the object is an extrusion it will not, by default, show isocurves.
Same in V5 and V6…
Is that it?
UseExtrusions is the command to switch to making polysurfaces if desired.
(just a reminder )
To stop extrusions being created when possible: Options > Rhino Options > General > Command Behavior > Uncheck “Use extrusions when possible”
Extrusions require less storage space and can result in smaller files. I don’t know of any other advantages of using extrusions over polysurfaces. Unless your project has a very large number of objects which could be extrusions there probably is no appreciable advantage to using extrusions.