My youngest is 11 months and we don't do baths every night simply because he continues to scream like he's drowning when he does no more than glance at the tub (or sink for that matter)
I do a little sponge bath every night before bed and I wipe him down after eating and changing his dipey, but the full baths are only every 3-4 days, sometimes *gasp* every 5 days. Keep in mind that he also has eczema, which is made so much worse by being in water for even a short time.
My 2 and 4 yr olds bathe nightly most of the time because they are dirty little creatures. Food everywhere, dirt in unmentionable places, hair that smells of pudding, you get the idea.
meg