After having about one million capacitive touch screened phones and tablets over the last few years, many of which have had screen protectors on, I have never experienced a reduction in sensitivity or accuracy when using a decent protector. I think Canon is merely covering itself from having to deal with people incorrectly applying protectors and hassling Canon over why the screens don't work perfectly any more.
That said - I've never managed to scratch a camera screen or a phone screen. So I wouldn't bother putting one on anyway