It can get by but it's always terrible without them all (Except harp since it's rarely used unless it's in a very large symphony). Without Violins, no high voice. No Violas? No cool mid range stuff, probably less interesting chords. No Cellos? A majority of the low voice is missing. Plus they sound great. No Basses? The backbone of the orchestra is missing. They sound beautiful if played correctly and when they get cool stuff, it is amazing.
Without them all the music is incomplete, PLUS professional orchestras don't need a conductor at all, if you're good you can do anything without one. The concert master (Principal Violin) controls the start of the piece and the Principal Bass determines tempo most of the time.