We do trim the cover not because it is 100% necessary, but because of the level of work we do here. Whenever we install something we tend to make it better than the way it comes. That goes for everything.
The install simply looks cleaner without the filter rubbing on the cover. Over time this will produce a visual wear pattern ont he end of the filter that some may not prefer.
That said, we never have to trim the cover as much as the picture you posted. On my own car you can not even visually tell that we cut it.
I agree, like I said you don't "have to", but it does hit, and it will wear over time.