I just like to have the option to choose fuel stations without worrying too much about having to stop "right now!".
Not so much for price, but to be able to pass on ones that are hard to get into, or say I'll just fuel up after we are settled in at the campsite.
We still stop way more often then the range on my tank, but that doesn't mean I want to try to combine every rest/stretch/bathroom stop into a fuel stop as well.
I just like the extra options a large tank provides.
It's like the argument about the new 8-10 speed transmissions. Will the trucks do the job without them...sure. Is having more options a bad thing? I don't think so!
I've considered installing a 25G side saddle tank from a Tahoe into my Suburban in addition to the stock 42G rear tank, with a switch to pump fuel over to the main tank.