You don't need fenders, but should definitely consider them
Also +1 for anyone saying full coverage is the way to go.

I inherited some cheap plastic fenders, that are not full coverage. Partial fender = partial coverage = some back(pack) splatter, but not full streak up back.
I removed by front fender, and for got to replace it.
If you enjoy facials a road oil/dirt slurry, I highly recommend not getting front fenders.