I'd assume the LFR's in a 5.1 will give you a similar sound field as you would get with a 7.1 setup using front presence speakers.
You could justify some of the difference by considering the cost of the potential reciever upgrade and the extra pair of surrounds.
After setting up two different 7.1 layouts I'd agree the effort and expense was worth it.
And...they're LFR's! Who doesn't want a pair or two?