LEt me say I DO NOT fish light drags, I fish minimum 130 jerry brown and 22 lbs at stike on 50's and 26 lbs on my 80's.
But when you fish smaller reels like 50's, and you fish light drag, that is when you really need the line capacity. When you preset a drag to 15lbs, your max drag is only around 28 i beleive which is nothing. IMO, if you wanna fish light drag mark your reel with tape, and set your preset higher and just don't push it to strike. This way if you have to go up you can.
I am very confident that on my 80's, I will never be spooled. I hope to hell I am proven wrong, but 38lbs on a full spool, 72lbs if you steel half of my 1300 yards, and towards the bottom thats almost 90lbs of drag!! That is also why I fish 200lb JB on my 80's!
Remember on a 50w, when 3/4 of the spool is gone your drag has doubled. So if you push past 20lbs and are getting dumped, you better hope your connections are 100%!
Do what works for you! But there is always a bigger fish swimming around, do you wanna be out gunned?