...i wouldn´t be as strict about that.
Modo has some pretty descent tools and workflows like a way of modelling SubD in Symmetry which is WAY more intuitiv than XSI´s way of doing that (clone, duplicate, -1 Scale, hide....stuff like that is anyoing) and also the renderer with its preview function is pretty handy.
Allthough i´m not considering a switch from XSI to modo i wouldn´t push that piece completely away, as it still is a non-autodesk-bound alternative if things become nasty...
I don't use that method of working in symmetry. I Ctrl A in poly mode to select all polys, then right click on selection to bring up ppg with "Symmetrize polygons". Now you can work with symmetry on till it breaks again...repeat process.
Symmetry breaks in all apps...even Modo.
The only alternative for me would be to move over to houdini.