How many of you have actually had a fish take you into the backing? If you primarily target species like bass, trout, and panfish, the answer may be never.
On the flip side, big, strong, far-running fish like tarpon, trevally, and salmon necessitate a reel that carries a big load of thin backing. The hottest examples of these fish can rip off a hundred yards or more of line, so a lot of quality, thin backing is crucial to both prevent getting spooled while also maintaining some "bulk" on the spool for decent retrieve speed when you need to crank all that line back in.
There's nothing wrong with filling up any reel with the rated amount of backing—but it can also be completely unnecessary. While it doesn't hurt anything from a fishing standpoint, you can save both time and even some money along the way by just filling certain reels with thick, heavy dacron line.
When I have fly reels sent to me for review here on Demystifly, they sometimes show up without backing. Since I'm bank-fishing 99% of the time, I'm testing these reels on fish that don't take a lot of line—usually largemouth bass, peacock bass, cichlids, snook, and smaller jacks that roam our beaches. For these fish, I simply do not need a lot of backing; I just need to bulk up the core of the reel's spool in the most efficient and cost-effective way possible.
I keep a 1000 meter spool of 50 pound braided dacron Magibraid line to fill up many of these review reels. It's cheap, smooth, won't break easily, and is obviously quite thick due to its high pound-test rating. The larger line diameter lets me put it on or take it off very quickly since any reel will hold far less yardage of this much thicker line. Despite the reduced capacity, larger reels will still hold some fair yardage in case I hook something that runs a bit further than expected. During my recent test of the new 7/8 Ross Gunnison, I hooked a chunky jack from the beach that promptly ripped off about a hundred feet of line. It didn't exactly go deep into my backing, but I certainly wasn't in any danger of being spooled.
If you have a reel or reels that are dedicated solely to fish that don't run very far, consider keeping a bulk spool of thick dacron line on-hand. I've had my 1000 meter spool of 50 pound for a couple years now, and even after all the reels I've filled up, I'm still nowhere near the end of it. I'm expecting it to last a couple more years! It sure makes life a lot easier not having to spend the time winding on 100+ yards of unnecessary backing all the time, and it'll save me some money in the long run as well. Just something to think about!