Not all rear racks are heavy.
Tubus racks made in Germany are considered the gold standard by many touring cycleist.
They make many styles. The "Airy" model is very light, 1/2# >
http://www.tubus.comI spend a good amount of time researching racks,panniers,bags. It seems the largest mis-conception is the weight of racks, rack bags,panniers,etc. Typically, it seems the average researcher of, looks at stout touring racks with heavy durable, highest quality touring panniers made for the long haul. There are light weight racks and ultra light weight panniers as well.
http://www.arkel-od.com/us/dry-lites.htmlA simple low cost, low weight option is to just use a dry bag,stuff sack,compression sack of about 15 L. size on top of the rack and be done with it. These bags can be very light weight if one wants >
http://www.rei.com/product/848438/outdoor-research-ultralight-compression-sack#tab-specs or a little heavier and more durable like mine below.
I have used a rear rack for a long time (25 years????). Believe me I spend a lot of time looking for other options........and my latest is frame pack > saddle bags.......but, other than pure race need and even then the longer the race (days versus hours) the less need for minor non rotational weight savings and the greater need for self suffciant cargo space........its tough to give up a rack.
Rack as a mud guard - here is my current set up. I run this townie mode to gravel tour,especially in winter (now).
This rack is a Planet Earth, about 1.5#, $30 bucks??? It doubles as a awesome rear mud guard with plenty of clearance (shown 1.8" tire ). I take a large saddle bag for tools (Topeka MTB large shown). It has rubber like tread on the bottom and straddles over the racks up-ward "L" tubing bend. A dry bags buckle then is buckle around the L bend of the rack and under the saddle tool bag. I then add a buckle strap around the bag. The dry bag shown is HUGE 30 L much larger than needed (i have several as I am a paddler/row'er also). I bolted on a mud flap to the rear and zip tied the mount for a rechargeable blinky rear light.
I have a inexpensive small saddle bag , commuter type pannier (16L total). It was a about $30 bucks>
http://www.backcountry.com/louis-garneau-explorer-b-16-cycling-panniersHere are a couple more photos of the same set up on my mutt different dry bag (little smaller 20L) of contrasting color so you can see the set up better.
I flop this on often in townie mode. They are not heavy touring panniers nor are they large.
It's really nice to have my tool saddle pouch separate.
Some act as if it is Such a hassle to take off a rack. I don't get that, its four small hex head bolts. I take mine off a lot during summer. Maybe 7 mins off/7 mins onetime?
Now, I am Not a "gravel racer" but I do ride a heck of a lot of gravel,dirt,fire/forest.
I have been eyeing up Revelate Designs fine stuff for some time. Its not like theirs weighs nothing as compared. In fact the one (not sweet roll the other) front bar set up is Really Heavy.
I am leaning towards a Revelate Designs Pika seat bag. There would be a little weight savings but, I also lose my mud guard function and a lot of versatility. So, I don't know ??? and there high priced ($135) for what they are.
BTW- I have back packs also. They sound great until you put even just 5#s in them and then they suck riding.
IMO - its between the bike packs and rack. I am not suggesting either as I am unsure myself and have not tried bike packs yet. But, I can share this - if your bike has braze ons for a rear rack and you happen to buy for $30-$50 bucks any rack from Blackburn, P.E. , OMM, etc............you will have a hard time taking it off and leaving it off..........I have for 25 years
Good Luck