+1. And as John N notes, you have additional capacity for those situations where you might need to pack extra water and food. During a tour across PA last year I had to carry lunch, dinner and breakfast food as well as snacks because there was nothing on route to my destination and nothing for about 15 miles the next morning.
A lot of this depends on how much you are carrying. I figure that 4 panniers start to make sense at somewhere above 30 pounds base gear weight (not counting food, water, or other consumables).
You do need to be able to carry enough to get by without resupply on some tours, so it is something to consider. Still even with very minimal baggage I have always been able to jam everything I needed in even when my bags seemed pretty tightly packed already. Worst case I have managed to carry 3 or 4 liters of water in jersey pockets. That was a little bit much but you use water pretty quickly so it was only a hassle for a short while. Taking along a little 2-1/2 ounce Sea2Summit backpack (or even a 12 ounce REI Flash 18) is a good solution for those rare days you need to go without services for a whole day and have some overflow. I actually find I like to wear it most of the time on some tours to hold the items I want to carry with me when off the bike as well as the items I will want at rest stops. I typically never have more than 2 pounds in it unless it is one of those all day and overnight without resupply days. It is nice to have along for day hikes and grocery runs.
At 215 lbs. riding a 60cm LHT, the additional weight of a front rack and two Sport Packers panniers is a very small percentage increase. It's even smaller when you factor in the weight of everything else.
This line of thinking can be a bad idea IMO, at least if you want to pack light. It is better too look at each decision based on the relative weights for that item only. If you use percentage of total load as the measure of whether an items is too heavy it can yield huge weight increases. There are many little choices and they add up even when each one is only a few ounces or even less. The a major portion of cutting my base weight from 45 pounds to 15 or less was cutting an ounce or a few ounces here and there. If you make 50 decisions that add only an ounce or two each you have added 5 or more pounds.
My advice is to carry what you need to be happy, but at the same time be diligent in weeding out unneeded items and heavier than necessary ones. I think this is a good policy for everyone whether heavy, medium, light, or ultralight packers.