You definitely need a clear filter:
I once had to clean my clear filter of my 24-105L in order to remove specks on it. They couldn't be removed by wiping, dusting and not using propanol. So, I figured it must be dust under the filter. Removed the filter. Still no luck with de-dusting, the specks were on the front side. So I tried to clean in an ultrasound bath using alcohol.
No luck, the specks were a damage of the coating or the glass of the filter by something agressive, perhaps salty water?? Never used the lens close to the sea, however. I lightheartedly threw the filter away the same day! No damage done to the lens! :-)
Bottom line: I always tried to be careful with my lens, but still I somehow destroyed the front filter... Had I destroyed the front element of my lens, I would have, I don't know what, but surely not simply replaced a comparatively cheap filter.
So, yes, one needs a front filter. Absolutely.