There's no harm in backing up your phone data, but with all of the OTA updates I've had across all my devices (3 on a Desire HD, 1 on a Wildfire S, 3 on an ASUS Transformer) , I've not lost any data. I'm not sure if ICS will be different, but I'd hazard a guess and say it won't be.
I could factory reset my phone right now and be back up to the way I had it in about 15 minutes, if that. Non rooted people are not as experienced, nor have the tools, to do the same.
