Yeah, i've run my track car for several years with no liners and never had any problems.
However, it does actually help a bit with aero. We probably don't notice the effects, but if you ever get into serious competition where every little bit may help, you may want to consider putting them (or something else) back in.
I've also never run/needed brake ducts. that being said I just put a splitter on and noticed that due to the lack of airflow underneath, the brakes seem to be affected more. I'll look at adding ducts for next season.